By Colin Packham

SYDNEY (Reuters) - A debate over Australia's notoriously tight gun laws has opened up a dangerous rift in the ruling Liberal Party, pitting Prime Minister Malcolm Turnbull against the man he overthrew ahead of a key party gathering this weekend.

Allegations of back-room dealing and broken promises to lift a ban on the importation of a lever-action shotgun have raised concerns that Australia's leadership revolving door - the country has seen four prime ministers in the past six years - is swinging back into action.

Turnbull, who seized power from former leader Tony Abbott in a party coup a year ago, is on the back foot after essentially accusing his predecessor under parliamentary privilege of lying on the issue.

At the heart of the row is Turnbull's decision to reject calls from farmer lobby groups to overturn a ban on the Adler shotgun for use in animal pest culling. Turnbull said on Friday he had no plans to water down Australia's tough anti-gun laws, introduced in the wake of 1996 Port Arthur Massacre where 35 people were killed.

"The ban, which I renewed, my government renewed, my cabinet renewed, will stay in place," he told reporters in Sydney.

But he has also accused Abbott of being part of a secret deal when he was leader to overturn the ban, a contentious move in a country where gun control is such a sensitive issue.

Abbott has made a series of now rare public appearances to refute the claims.

The public disagreement between the two men is likely to be more damaging for Turnbull, who has already seen his poll numbers sink to all time low, as Abbott continues to enjoy support among the conservative wing of the party.

"The Australian government has mostly been mired in its own internal problems and this row allows that to dominate and it will further alienate the public," said Peter Chen, senior lecturer in politics at the University of Sydney.

Defence Industry Minister Christopher Pyne, who supported Turnbull in the leadership coup, tried to play down the dispute but it is likely to dominate this weekend's party conference in New South Wales.

It is also expected to damage Turnbull's legislative agenda, notably the passage of a flagship bill to stamp out corruption in the building industry.

Independent Senator David Leyonhjelm, who has a crucial swing vote, is tying his vote to the Adler shotgun.

"If the ban remains in place, it will be a point of contention between myself and the government for the next three years," Leyonhjelm told Reuters.

(Editing by Jane Wardell and Nick Macfie)