THE most significant conscience vote in State Parliament in decades - whether to allow gay couples to adopt - is dividing major parties and led to bizarre claims it would increase the number of abortions.

While committed Catholic Premier Kristina Keneally will support the Bill to allow same-sex adoption, Opposition Leader Barry O'Farrell and most senior Liberals yesterday refused to reveal their stance.



Mr O'Farrell's spokesman said that was because he did not want his view influencing his MPs. Opposition treasury spokesman Mike Baird said: "I can't put my hand on my heart and say it's a good thing to deny future kids the chance for a mum and dad."



Senior ministers, including Deputy Premier Carmel Tebbutt and Treasurer Eric Roozendaal, will support the Bill.



At a rally against the Bill yesterday, Liberal Party powerbroker David Clarke and Christian Democrat Fred Nile warned abortions would increase if gay couples were allowed to adopt.



"Any mother putting up her baby for adoption would never imagine that their baby would be brought up by two male homosexuals or two female lesbians," Mr Nile claimed.



Questioning whether Ms Keneally allowed a conscience vote to win Greens preferences at next year's election, Mr Clarke said: "The optimal position is when a child, where possible, is brought up by a mother and father. We know that always can't be done ... but that's the optimum."



The Bill - introduced by Sydney MP Clover Moore - is expected to be debated today, with all MPs granted a conscience vote for the first time on the issue.



Ms Keneally's former leadership rival Frank Sartor is trying to amend the Bill to ensure people who offer children for adoption will not face legal action under anti-discrimination laws if they refuse to let a gay couple adopt their child. His office yesterday said he was "undecided" on how he would vote.



If successful, Education Minister Verity Firth said the Bill would remove the last law in NSW that discriminates against same-sex couples.



It would also make NSW one of three states, joining Western Australia and ACT, allowing gay couples to adopt.



The Benevolent Society charity and UnitingCare Burnside, one of the largest child and welfare agencies in NSW, supported the Bill, saying it would benefit children and boost foster carer numbers.