Nationals MP Llew O'Brien is set to join his Queensland Nationals colleague George Christensen and vote in the lower house for the banking commission of inquiry after the revolt was kick-started by Queensland senator Barry O'Sullivan.

But the head of the recent financial system inquiry, David Murray, warned a banking inquiry without safeguards could veer into "dangerous territory" while CLSA bank analyst Brian Johnson said it could affect the home loan market in the medium term.

Nationals sources said it was possible the party could, next Monday, decide to allow a free vote on the issue, and that if this happened more MPs could come forward and vote for the commission of inquiry.

Alternatively, the Nationals could split from the Liberals and formally switch position as a party to back the probe.

Having told Fairfax Media 10 days ago he was a 50-50 chance to cross the floor, Mr O'Brien locked in support for the inquiry after securing amendments to the bill proposed by Senator O'Sullivan that will broaden the terms of reference so insurance companies that have discriminated against people with mental health issues is examined.

He told Fairfax Media on Monday "there has been no reason put to me yet that would stop me crossing the floor".

Mr O'Brien's predecessor in the seat of Wide Bay, former deputy prime minister and Nationals leader Warren Truss, told Fairfax Media that he did not support a banking inquiry as "it will cost $100 million or more, it will give people a chance to have their grievances heard, but it won't make out system better".

The growing likelihood of an inquiry comes amid Coalition infighting over the Queensland state election result, which saw the Liberal National Party bleed votes to One Nation and Mr Christensen blaming, in part, the federal government - and the Prime Minister - for the result. His Queensland LNP colleague Warren Entsch hit back at him for "always firing shots".

If the inquiry goes ahead with the support of the Parliament, thereby overriding the wishes of the government, it will be another damaging blow to Mr Turnbull's standing and authority in the Coalition.

Another Nationals MP, Victorian Andrew Broad, has also told Fairfax Media he could back the inquiry.

"I will listen to debate in the Nationals party room and then make a decision," he said, though he added some of the angst in his state had dissipated because the former Coalition state government had established farm debt mediation to deal with complaints about the banks.

Mr Christensen on Monday also launched a banking inquiry petition and website that outlined 10 reasons why an inquiry is needed.

Former Nationals leader Barnaby Joyce, who has opposed an inquiry and who is currently campaigning to win back his seat of New England, signalled a broader shift from the junior Coalition partner was in prospect.

"You can take anything to the Nationals party room for consideration. We will discuss it [the inquiry] and come to a decision, we will have the discussion after the byelection," he said.

Senator O'Sullivan's bill, released last week, is set to sail through the Senate with the support of the Nationals, Greens, Labor and some on the crossbench.

But to be passed in the lower house, 76 votes are needed to suspend standing orders and debate the bill; Mr O'Brien and Mr Christensen's two votes - combined with 74 from Labor and the crossbench - would deliver the numbers.

A simple majority is then needed to pass the bill and set up the inquiry, which would report to the Parliament, not the executive, but have the same powers as a royal commission.

Senator O'Sullivan's wide-ranging inquiry would appoint three commissioners - a former judge, a financial expert and a community representative - and run for three years.

He suggested Mr Turnbull still had a chance to intervene, take charge of the process and order a royal commission

"Clearly an inquiry led by government would be superior to one that was forced on it by the numbers on the floor of two houses. I have been and remain open to that happening," Senator O'Sullivan said.

Mr Murray said he did not think a banking inquiry was necessary and that, for example, any recommendation to reduce the level of the banks' vertical integration would be bad for competition.

"And with the financial system and its effect on the economy, it's dangerous territory," he said.

Treasurer Scott Morrison is meanwhile working on a "last resort" compensation scheme for people who have been ripped off by banks and other financial institutions, in a last-ditch move to head off the Nationals push.

The details of that scheme could be announced as soon as this week.