Retiring Liberal MP John Alexander has joined his colleague Bridget Archer to say he will vote against his own party to legislate a federal corruption watchdog before the election.

The two Liberal MPs now stand alongside crossbench MPs and Labor in pushing for a federal integrity commission to be legislated, which the government has conceded it has run out of time to do.

However, the rebellion is unlikely to force the government's hand because complex voting arrangements set up to minimise the risk of COVID-19 infections leave them unable to reach the absolute majority needed for a vote on an integrity commission to succeed.

Mr Alexander said that, before he left parliament for good, he would like to see action on creating a corruption watchdog.

"It is time that we stop bashing heads and put our heads together," he said.

"A very fundamental thing [about] our representatives is we have to be able to believe in them. We have to be able to trust them that they are telling the truth. That they have integrity.

Ms Archer shocked government benches last year when she moved to support independent Helen Haines's motion to bring on debate for an integrity commission.

The Coalition narrowly avoided embarrassment in parliament thanks to a COVID-19 technicality, managing to block the attempt.

Attorney-General Michaelia Cash has admitted that the government has run out of time to legislate on the issue, despite promising to do so at the last election.

The government argues it has a bill to enshrine a federal integrity commission ready, but will not introduce it because it does not have the support of Labor or the crossbench.

The commission proposed in the bill has been widely criticised as lacking "teeth" and the powers needed to properly investigate and make findings on allegations of corruption, as well as being too narrow in its remit.

Today, Ms Archer urged the Coalition to allow a debate to progress plans to establish a watchdog.

"It's so important," she said. 

"It's the most important thing we do.

"This issue must be beyond politics. If it's not beyond politics, if we can't come together and collaborate, this will never be done."

Greens MP Adam Bandt noted that when a bill was put to the Senate, it brought together politicians of all stripes — a rare occurrence.

"If the government let us have a debate, we could have a corruption watchdog with teeth before the election," he said.

"A bill has already passed the Senate that was moved by the Greens, supported by Labor, independents and One Nation.

"We were here until 5am last week debating a bill the government thought was important. I'm happy to stay back till 5am tonight debating an integrity commission."