THE Opposition Leader, Malcolm Turnbull, sacked one frontbencher for dissent yesterday while standing by his shadow treasurer, Julie Bishop, who faces unsourced criticisms of not being up to the job.

In a turbulent day for the federal Coalition, the NSW Nationals senator Fiona Nash was asked by Mr Turnbull to stand down as shadow parliamentary secretary.

Senator Nash and her three National Party colleagues were all due to cross the floor last night to vote against the Liberal Party on a Senate motion demanding an overhaul of a bill that pays tax deductions for the planting of trees as carbon sinks.

Labor and the Liberals combined to defeat the disallowance motion but the Nationals were planning to side with the Greens. Their concern is that the generous tax break distorted land prices and undermined food security by using valuable arable land.

Mr Turnbull played down the Coalition split in advance, telling ABC radio yesterday morning: "We're not like the Labor Party, we're a broad church and people are entitled to … take a particular position if you like, on a matter of conscience."

The Environment Minister, Peter Garrett, accused Mr Turnbull of being unable to unite his party on climate change.

Mr Turnbull stood by Ms Bishop who is fighting unsourced reports her colleagues want her dumped, while waging an open war with the West Australian academic and political author Peter van Onselen. He has been critical of Ms Bishop since she was accused of plagiarism when preparing a chapter for a book he edited on the future of the Liberal Party.

"There seems to be an Edith Cowan University academic who is developing, shall I say, an unhealthy obsession with me," Ms Bishop said.

Mr Turnbull defended her against claims, reportedly from colleagues, saying "she's doing a great job" and suggesting the focus shift to the Treasurer, Wayne Swan. A senior frontbencher said there was no prospect of a reshuffle and no one was about to sack Ms Bishop.