The 38-year-old won the Victorian seat of Hotham, in Melbourne's leafy east, after she was a last-minute replacement for local councillor Geoff Lake.

Prime Minister Kevin Rudd stepped in to dump Mr Lake as a candidate just before the election, after comments he made about another councillor 11 years earlier resurfaced.

Labor had to pick a new candidate days before nominations closed, and the then 32-year-old was flung into the fray.

As the youngest woman to be elected as a mayor in Australia's history, Ms O'Neil was a rising star in Labor circles.

She joined the party at 16 and the first MP she met was Simon Crean, whose seat of Hotham she took in 2013.

At 23 she was elected mayor of the City of Greater Dandenong when her fellow councillors picked her in a ballot.

"I think they recognised it's good for the city to have something a bit fresh and a bit different," she said in March 2004.

Ms O'Neil studied at Harvard in the United States on a scholarship, and later worked for management consultancy firm McKinsey and Company.

She also spent nine months living in Arnhem Land in the Northern Territory in 2011.

In her first speech to parliament, she talked about how her grandparents were held back from success by economic disadvantage.

"For the growing parts of our economy, the old lines of labour and capital have all but disappeared," she said in September 2013.

"Working people - more than two million of whom now own their own businesses, and almost all of whom are shareholders - know that a fair society needs a strong economy, and that means businesses that work.

"The big thinkers in Australian business also know that government is essential to helping them succeed."

Ms O'Neil was named Labor's justice spokeswoman after the 2016 election, and was promoted to the financial services portfolio in June 2018 as the banking royal commission was taking off.

She pushed for the royal commission to be given more time to hear from victims, and for lawyers to help people who banks have ripped off.

Before she was on the Labor front bench, Ms O'Neil introduced a private members bill into parliament in a bid to ban the importing of products tested on animals.

As one of the few prominent women in the lower house from Labor's Right faction, Ms O'Neil is now in the frame for a senior role.

Australian Associated Press