The by-election in the western Sydney seat of Blacktown has been caused by the retirement of John Robertson, the member since 2011. Robertson rose to prominence within the Right faction as secretary of Unions NSW from 2001, then entered parliament in the Legislative Council in October 2008. He was promoted to cabinet the following year and began casting around for a lower house berth, with a view to assuming the leadership following Labor's inevitable defeat. The required vacancy duly became available when Paul Gibson announced his retirement in Blacktown four months out from the 2011 election, and Robertson duly became leader of the shattered party after the election.
Robertson's hold on the leadership became increasingly precarious as the 2015 election loomed, and he resigned in December 2014 after it emerged he had sent a letter to the Department of Family and Community Services on behalf of Man Haron Monis, who went on to conduct the Lindt café siege. He spent the remainder of his parliamentary career on the back bench, and announced his resignation in August to take up a position as executive general manager at Foodbank, an organisation that distributes excess food from retailers to the needy.
Blacktown covers suburbs around 30 kilometres to the west of central Sydney, with boundaries set by the Westlink M4 in the north and the Great Western Highway in the south. It encompasses Blacktown itself and its western neighbour Doonside, together with Kings Park, Marayong and southern Quakers Hill to the north. Since its creation in 1941, Labor has won the seat at every election except 1959, at which time its electoral boundaries briefly extended into Sydney’s northern outskirts.
Blacktown was one of only 20 seats that stayed with Labor in 2011, when John Robertson held on by 3.7% in the face of an 18.7% swing, followed by a 9.0% swing back to Labor in 2015. Robertson's immediate predecessor was Paul Gibson, a colleague of Eddie Obeid and Joe Tripodi in the Terrigals sub-faction of the New South Wales Right, and an enormously contentious figure in his own right. Gibson in turn came to the seat in 1999, after previously holding the seat of Londonderry since 1988.
Labor's candidate for the by-election is Stephen Bali, the mayor of Blacktown since 2015 and a former assistant state secretary of the Right faction Australian Workers Union. Bali won preselection with Robertson's endorsement, apparently without opposition.