After five years Peta Murphy thought she was in the clear. But a shock new cancer diagnosis won't stop her from making a defiant first speech to the House of Representatives next week.

The 45-year-old Labor MP, who won the south-east suburban Melbourne seat of Dunkley at the May 18 election, has vowed her "fighting spirit" will get her through.

Labor MP Peta Murphy will undergo treatment for cancer. Credit:Darrian Traynor

Sworn in as a new MP just 15 days ago, she received the "unexpected news" last week that her cancer had returned.

"While this has come as a shock, my doctor advises me that my condition is treatable and that he expects me to do well with treatment, which I will start in the coming weeks," Ms Murphy told The Sydney Morning Herald and The Age.