At the February 2001 state election, where the Labor party won government, MacTiernan became Minister for Planning and Infrastructure, a portfolio which included Transport, in the new Gallop Ministry.
She held this position until Labor's defeat at the August 2008 state election, and was best known for overseeing of the construction of the Mandurah railway line, which she had championed from opposition.
[2] She officially resigned from state parliament on 19 July 2010, two days after Prime Minister Julia Gillard had announced the timetable for the 2010 federal election.
[3][4][5] On 13 August 2010, the Speaker of the Western Australian Legislative Assembly issued a writ directing the Electoral Commissioner to proceed with an election in the district.
[6] Tony Buti, a law professor at the University of Western Australia and chairman of the Armadale Redevelopment Authority, won the Labor preselection unopposed.