Porter began her acting career in House Gang, a short lived television program. She was in one episode and was on air only for a limited time. She then had small roles in other Australian television programs in the late 1990's, including Big Sky, Wildside and Water Rats. In 2000, she starred in the film Bootmen. In the 2000 crime drama film The Monkey's Mask, Porter plays a lesbian private detective who falls in love with a suspect. In 2001 she appeared in the Australian movie Mullet and had a small role in Star Wars Episode II: Attack of the Clones.

Porter has received five nominations by the prestigious Australian Film Institute. In 1999 she was nominated for Best Performance by an Actress in a Supporting Role for Two Hands with Heath Ledger and Bryan Brown. In 2000 she was nominated for Best Performance by an Actress in a Lead Role for Better Than Sex co-starring David Wenham. 2003 saw Porter nominated for Best Actress in a Lead Role for the low-budget Teesh and Trude.

In 2005, Porter had a supporting role in in the Australian film Little Fish.

2006 was successful year for her with a leading role in RAN, an Australian mini-series, which won her a 'Best Leading Actress in a Television Drama' at the AFI Awards and she also had a role in the film The Caterpillar Wish which won her Best Supporting Actress in the AFI Awards.

In 2007, she won the Most Outstanding Actress at the Logies for her role in Remote Area Nurse.