Malcolm Turnbull’s prime ministership may be entering a second, more difficult phase as Mal Brough and Ian Macfarlane threaten to bring an end to the new PM’s three-month honeymoon.

Just days after Mr Brough survived two weeks of withering attacks during question time, leading the Opposition to question Mr Turnbull’s judgment, the PM late on Sunday withdrew his planned attendance from a Toowoomba event with Mr Macfarlane following the Federal Member for Groom’s indication he wants to switch to the Nationals.

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Mr Macfarlane’s decision to desert the Liberal partyroom for the junior Coalition partner threatens to trigger a cabinet reshuffle and regain his place on the frontbench after being demoted when Mr Turnbull was elevated to the prime ministership.

It has also caused a high degree of consternation from some of his colleagues.

Attorney-General George Brandis said Mr Macfarlane should not be allowed to force a cabinet reshuffle by “gaming the system” by switching parties.

The former Abbott government minister announced his decision to move to the Nationals in an attempt to reclaim a position on the frontbench.

But Senator Brandis said a lot needed to happen before Mr Macfarlane could realise his ambition.

He will need the approval of the Queensland LNP state executive to approve his switch at a meeting in a week’s time.

“No backbencher can force a cabinet reshuffle on a prime minister by swapping parties in order to game the system,” the Attorney-General told Network Ten on Sunday.

Suggestions that Resources Minister Josh Frydenberg could be replaced by Mr Macfarlane was “unthinkable”, he added.

Labor leader Bill Shorten believes the saga is the first visible fault line in a “bitterly divided” government.

The prime minister will be put in a difficult position to reward such “undermining” when the Nationals push for another cabinet seat.

“That’s a Coalition in crisis,” Mr Shorten told ABC TV on Sunday.

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.

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.”

– with AAP, ABC