At the 1993 election, she moved to the Legislative Assembly, successfully contesting the seat of Armadale and in doing so replacing Bob Pearce whose career had effectively been ended by the WA Inc royal commission. She served as Deputy Opposition Leader from February 1993 until October 1994. She retired from parliament at the 1996 election, being succeeded by Alannah MacTiernan (who had also succeeded her in the East Metropolitan seat in the Council in 1993).
In 2002, she was made an Officer of the Order of Australia (AO) for "service to the community, particularly as an advocate for the needs of children, women and the elderly, in matters of social justice, and to the Parliament of Western Australia."[2] She also became chairperson of Save The Children Australia. At the 2004 federal election, following the death of one preselected candidate and the resignation of her replacement, Hallahan nominated to run for Liberal-held Division of Canning, which included Armadale, but was defeated. Ironically Alannah MacTiernan, who had succeeded Hallahan in her Legislative Council and then Legislative Assembly seats, also stood for the ALP in Canning at the 2010 election but she too was unsuccessful. However unlike Hallahan, MacTiernan eventually found her way to Federal Parliament when she won the seat of Perth at the 2013 election.
