Prime Minister Malcolm Turnbull has withdrawn from a Toowoomba event with Ian Macfarlane after the Federal Member for Groom said he wanted to switch to the Nationals.

Instead, Deputy Prime Minister and Nationals leader Warren Truss will join Mr Macfarlane to turn the first sod on the much-anticipated second Toowoomba range crossing.

The Prime Minister's decision comes after senior Queensland Liberals cast doubt on Mr Macfarlane's switch to the National partyroom, citing the need for endorsement from the party executive.

Mr Macfarlane, a former Abbott minister, wants to swap from the Liberal to the National party rooms following his demotion from the frontbench in September.

Key points: Ian Macfarlane wants to swap from the Liberal to the National party following his demotion from the frontbench

Ian Macfarlane wants to swap from the Liberal to the National party following his demotion from the frontbench Nationals state executive has not endorsed the switch as yet

Nationals state executive has not endorsed the switch as yet Peter Dutton, George Brandis among ministers to doubt the party switch

The move has been welcomed by some senior Nationals, but he is yet to gain endorsement from the party's state executive.

Queensland Liberal frontbencher Peter Dutton said Mr Macfarlane's move was not a fait accompli simply through his Thursday announcement.

Mr Dutton told Sky News that the state executive, expected to meet later this month, would have "something further to say".

"[Macfarlane's] conducted an operation which is behind the scenes and I think the state executive of the LNP in Queensland will contemplate whether that's going to be a feasible step," he said.

"This issue hasn't been resolved."

Queensland Liberal Attorney-General George Brandis has expressed similar doubts, telling Channel Ten there could be upset over the fact Mr Macfarlane did not reveal his planned defection a fortnight ago when he was endorsed for the 2016 election.

"We now know that this plan [to defect] had been in being for weeks before that," he said.

"And I think that, frankly, the branch members of the LNP who endorsed Mr Macfarlane on that understanding are entitled to ask him why he didn't reveal to them that he had these plans in place at the time he sought their endorsement to sit as a Liberal in the Federal Parliament."

Mr Macfarlane's move also gives the Nationals a chance at a fourth Cabinet seat, based on a formula agreed upon by the Liberal and National Parties.

Macfarlane demotion 'bites Turnbull on political bottom': Shorten

Senator Brandis said the National Party was entitled to its share of ministries, but questions remained over Mr Macfarlane's methods.

"No backbencher can force a Cabinet reshuffle on a prime minister by swapping parties in order to game the system," he said.

"I don't believe that Mr Macfarlane should have done what he did ... but there's quite a lot more to happen before Mr Macfarlane's ambition can be given effect to."

Opposition Leader Bill Shorten has described the announcement as the first visible fault line in the Coalition.

Mr Shorten told the ABC that the "Macfarlane flip" leaves Mr Turnbull in a tough position.

"Is he going to give in to this stunt, to this undermining, reward it with a new Cabinet spot and that of course is going to make all the younger Liberals very angry, or does he say no, then you've got a Coalition in crisis?" he said.

He said Mr Turnbull's demotion of Mr Macfarlane had "come back to bite him on the proverbial political bottom".

"It clearly is embarrassing," he said.

"If he knows it's going to make Malcolm Turnbull angry and it clearly has — Malcolm Turnbull pulled out of a Macfarlane function over the weekend — and then he still does it, there's a real issue there."