Lake Macquarie independent MP Greg Piper says he will find it "very difficult" to keep supporting the NSW government if its new leader does not allow Liberals a conscience vote on euthanasia.
Mr Piper has co-authored a voluntary assisted dying bill which fellow independent Alex Greenwich will table in Parliament on October 14.
Departing Premier Gladys Berejiklian has not backed the legislation but was expected to allow colleagues a conscience vote on the issue.
Her likely successor, Treasurer Dominic Perrottet, is a staunch Catholic and conservative and heads the Liberals' right faction.
"It would be the dog being wagged by a very conservative tail," he said on Monday, the day before the Liberals elect a new leader.
Mr Piper, Mr Greenwich and fellow independent Joe McGirr have helped guarantee the minority coalition government supply and confidence during a term beset by scandals and resignations.
The Liberals and Nationals hold 46 of the 93 lower house seats and have former Liberals John Sidoti and Gareth Ward on the crossbench.
Ms Berejiklian, Transport Minister Andrew Constance and Nationals leader John Barilaro have all announced their resignations in the past four days, leaving the government facing three by-elections before the next NSW election in 2023.
Ms Berejiklian ruled out more conscience votes in this term of government after a fractious abortion debate in 2019, but Mr Perrottet may not be bound by that pledge.
Sources said he would allow a conscience vote on the new bill, as have the Nationals and Labor, though the issue is causing a good deal of hand-wringing in the Liberal party room.
Mr Constance, before announcing his intention to quit state politics and run for the federal seat of Gilmore, told his caucus colleagues last week that he would cross the floor if he was denied a conscience vote.
Mr Piper, who had a close working relationship with Ms Berejiklian, upped the pressure on Mr Perrottet on Monday, describing voluntary assisted dying as a "die in the ditch issue".
"Discussions will become fairly tense if a conscience vote is not on the table," he said.
"It's a peek into the nature of the government."
The bill says only adults diagnosed with a terminal illness that will cause death within six months, or 12 months for neurodegenerative conditions, can access voluntary assisted dying.
Two experienced doctors would need to conclude independently that the patient is acting voluntarily and without pressure.
Mr Greenwich released a draft of the proposed laws in July, almost two years after an assisted dying bill failed by one vote in the upper house.
Mr Piper said at the time that previous attempts to introduce the laws had failed because "a number of parliamentary leaders have allowed their religious beliefs to outweigh the wishes of the vast majority in their communities".