This is a list of by-elections for the New South Wales Legislative Assembly. A by-election may be held when a member's seat becomes vacant through resignation, death or some other reasons. These are referred to as casual vacancies. {| class="wikitable"

Fifty-eighth Legislative Assembly 2023–2027

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Fifty-seventh Legislative Assembly 2019–2023

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Fifty-sixth Legislative Assembly 2015–2019

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Fifty-fifth Legislative Assembly 2011–2015

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Fifty-fourth Legislative Assembly 2007–2011

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Fifty-third Legislative Assembly 2003–2007

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Fifty-second Legislative Assembly 1999–2003

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Fifty-first Legislative Assembly 1995–1999

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Fiftieth Legislative Assembly 1991–1995

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Forty-ninth Legislative Assembly 1988–1991

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Forty-eighth Legislative Assembly 1984–1988

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Forty-seventh Legislative Assembly 1981–1984

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Forty-sixth Legislative Assembly 1978–1981

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Forty-fifth Legislative Assembly 1976–1978

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Forty-fourth Legislative Assembly 1973–1976

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Forty-third Legislative Assembly 1971–1973

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Forty-second Legislative Assembly 1968–1971

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Forty-first Legislative Assembly 1965–1968

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Fortieth Legislative Assembly 1962–1965

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Thirty-ninth Legislative Assembly 1959–1962

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Thirty-eighth Legislative Assembly 1956–1959

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Thirty-seventh Legislative Assembly 1953–1956

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Thirty-sixth Legislative Assembly 1950–1953

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Thirty-fifth Legislative Assembly 1947–1950

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Thirty-fourth Legislative Assembly 1944–1947

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Thirty-third Legislative Assembly 1941–1944

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Thirty-second Legislative Assembly 1938–1941

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Thirty-first Legislative Assembly 1935–1938

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Thirtieth Legislative Assembly 1932–1935

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Twenty-ninth Legislative Assembly 1930–1932

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Twenty-eighth Legislative Assembly 1927–1930

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Twenty-seventh Legislative Assembly 1925–1927

The 25th, 26th and 27th Legislative Assemblies were elected using a form of proportional representation with multi-member seats and a single transferable vote (modified Hare-Clark). Under the provisions of the Parliamentary Elections (Casual Vacancies) Act, casual vacancies were filled by the next unsuccessful candidate on the incumbent member's party list. If an Independent member retired, the Clerk of the Assembly determined who would fill the vacancy based on the departing members voting record in questions of confidence. The date listed as the by-election date is the day on which the new member was sworn into the Assembly. !Electorate !! Incumbent !! colspan=2|Party !! Reason for vacancy !! Date of appointment !! Person appointed !! colspan=2|Party ¶ Kay had supported the Lang Government in votes of confidence in the Assembly, the Clerk of the Parliament therefore named the first unsuccessful Labor candidate in the North Shore electorate as his replacement.

Twenty-sixth Legislative Assembly 1922–1925

!Electorate !! Incumbent !! colspan=2|Party !! Reason for vacancy !! Date of appointment !! Person appointed !! colspan=2|Party

Twenty-fifth Legislative Assembly 1920–1922

!Electorate !! Incumbent !! colspan=2|Party !! Reason for vacancy !! Date of appointment !! Person appointed !! colspan=2|Party ¶ The Speaker had received correspondence nominating two unsuccessful candidates from the 1920 election, Thomas Hynes and John O'Reilly. However, the terms of the Parliamentary Elections (Casual Vacancies) Act stated that a nomination to fill the vacancy had to come from a recognised party leader. Labor leader and Premier John Storey nominated Jabez Wright. ‡Millard had been appointed to replace Gus James and, as there were no further unsuccessful Nationalist candidates, the Parliamentary Elections (Casual Vacancies) Act was amended to allow his replacement by another Nationalist supporter.

Twenty-fourth Legislative Assembly 1917–1920

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Twenty-third Legislative Assembly 1913–1917

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Twenty-second Legislative Assembly 1910–1913

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party ¶ Mudgee Labor MLA Bill Dunn and Liverpool Plains Labor MLA Henry Hoyle resigned from the Labor Party and Parliament on 25 July 1911 in protest at legislation on land ownership introduced by Lands Minister, Niels Nielson. As a result, Labor was left without a majority in the house and rather than face a vote of no confidence, the Ministry and Speaker resigned. This forced the House to be prorogued with the result that Cobar Labor MLA, Donald Macdonell, who was unwell but expected to recover, was automatically expelled for non-attendance during an entire session. By the time of the subsequent by-elections, Labor policy had been reversed and Nielsen had left the ministry. Dunn rejoined the party and successfully re-contested, the Mudgee by-election on 16 August 1911. Hoyle did not re-contest the Liverpool Plains by-election which was won by Liberal candidate, John Perry by three votes on the same day. This result was overturned on appeal and at the second by-election on 28 October, Labor candidate, William Ashford was successful. In Cobar, Macdonell was unopposed when re-elected on 7 October, but died three weeks later. The Labor candidate, Charles Fern was unopposed at the second by-election on 16 December. Denied further ministerial appointment Nielsen resigned from the seat of Yass and Labor candidate Greg McGirr won the resultant by-election on 19 July 1913.

Twenty-first Legislative Assembly 1907–1910

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Twentieth Legislative Assembly 1904–1907

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Nineteenth Legislative Assembly 1901–1904

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Eighteenth Legislative Assembly 1898–1901

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Seventeenth Legislative Assembly 1895–1898

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Sixteenth Legislative Assembly 1894–1895

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Fifteenth Legislative Assembly 1891–1894

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Fourteenth Legislative Assembly 1889–1891

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

Thirteenth Legislative Assembly 1887–1889

!By-election !! Incumbent !! colspan=2|Party !! Reason !! Date !! Winner !! colspan=2|Party

No party system was discernible in the New South Wales parliament before the election of 1887

{| class="wikitable"

Twelfth Legislative Assembly 1885–1887

! By-election ! Incumbent ! Reason ! Date ! Winner

Eleventh Legislative Assembly 1882–1885

! By-election ! Incumbent ! Reason ! Date ! Winner

Tenth Legislative Assembly 1880–1882

! By-election ! Incumbent ! Reason ! Date ! Winner

Ninth Legislative Assembly 1877–1880

! By-election ! Incumbent ! Reason ! Date ! Winner

Eighth Legislative Assembly 1874–1877

! By-election ! Incumbent ! Reason ! Date ! Winner

Seventh Legislative Assembly 1872–1874

! By-election ! Incumbent ! Reason ! Date ! Winner

Sixth Legislative Assembly 1869–1872

! By-election ! Incumbent ! Reason ! Date ! Winner

Fifth Legislative Assembly 1864–1869

! By-election ! Incumbent ! Reason ! Date ! Winner

Fourth Legislative Assembly 1860–1864

! By-election ! Incumbent ! Reason ! Date ! Winner

Third Legislative Assembly 1859–1860

! By-election ! Incumbent ! Reason ! Date ! Winner

Second Legislative Assembly 1858–1859

! By-election ! Incumbent ! Reason ! Date ! Winner

First Legislative Assembly 1856–1858

! By-election ! Incumbent ! Reason ! Date ! Winner

Causes

A by-election may occur whenever there is a vacancy in the Legislative Assembly. Vacancies can occur for reasons including: It is now a convention that a by-election is not held if a vacancy occurs within 3–4 months of an expected dissolution of the parliament.