Attorney-General George Brandis says Ian 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.
Senator Brandis is the latest in a line of Liberals to criticise Mr Macfarlane, some who accused him of blatant self-interest.
Mr Macfarlane's desire to move to the Nationals had "left a very bad taste in people's mouths", the attorney-general said.
The former minister had questions to answer about why he sought Liberal preselection for the seat of Groom when he had plans to leave the party.
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.
Cabinet minister Peter Dutton disagreed, saying it did not dilute Prime Minister Malcolm Turnbull's authority.
"He is very respectful of the coalition, very determined to make sure that relationship works and works well," Mr Dutton said.
Mr Macfarlane was dumped as a minister when Mr Turnbull rolled former prime minister Tony Abbott in September.
Mr Turnbull declined to speculate on the prospect of a cabinet reshuffle.
"As far as internal matters are concerned ... I will discuss them with my colleagues whether it's in the National party, or the Liberal party or the LNP," he told reporters in Sydney.
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