The Turnbull government has suffered an embarrassing blow to its narrow majority after it lost three consecutive votes in the lower house.
It's believed to be the first time in five decades that a majority government has lost a vote in the House of Representatives.
Labor exploited a weakness in the government's numbers late Thursday afternoon when many MPs were leaving Canberra for home.
The opposition tried to bring on debate in a bid to call on Prime Minister Malcolm Turnbull to establish a royal commission into banks.
Labor then won three divisions in its attempt to expedite debate, taking advantage of the coalition's slender majority.
Speaker Tony Smith was also forced to use his casting vote - believed to be for the first time - on a procedural motion to allow the debate to be considered.
Treasurer Scott Morrison denounced the move and accused Labor leader Bill Shorten of taking political advantage.
"He is using a stunt to promote his stunt," he told parliament.
Mr Shorten said the royal commission was the last resort for justice for those who'd been let down by the banking system.
"We may succeed tonight or not .... we will never give up."
Shadow treasurer Chris Bowen urged the prime minister to give up the fight against a royal commission.
"Sometimes you just have to know when you've lost - sometimes you just have to know when it's time to recognise reality ... when it's time to say 'OK I might have got this one wrong'."
Other Labor MPs were celebrating the victory on social media.
"How is that stable majority government working out for you, Mr Turnbull?" frontbencher Mark Dreyfus tweeted.
Former prime minister Tony Abbott says the Turnbull government has lessons to learn after it suffered an embarrassing vote loss in the lower house.
The opposition, in celebration mode over the victory, said it took just two sitting days for the government's majority to collapse.
The coalition was on notice.
