Ms Berejiklian has previously indicated her support for same-sex marriage and recently attended the launch of an event for Liberal and National party voters who were voting 'yes' in the forthcoming national postal plebiscite on marriage equality.

But at a fundraiser event at NSW Parliament for Australians for Marriage Equality on Tuesday night she made her strongest public statement on the issue to date.

"I'm extremely proud to be here as the Premier of NSW," she said. "I'm hoping that all of us will look back and feel really proud that we were part of something special [and] putting aside our political differences in support of what is one of the most important decisions of our time in terms of human rights and social justice."

Ms Berejiklian, who is a member of the Armenian Orthodox faith, has previously staked out progressive stances on social issues and counts among her closest confidants North Sydney MP Trent Zimmerman, one of the strongest supporters of the 'yes' campaign in the Federal Coalition party room.

"For us it is a no-brainer, but for many in the community it won't be so simple," Ms Berejiklian told a crowd that included senior members of her cabinet and other state MPs active in the Australian Marriage Equality campaign including Alex Greenwich, independent member for Sydney.

"It is our job to ensure that, as respectfully as possible, we explain why it is so critical that we as a society move forward to join ... pretty much every English-speaking nation.

"For me the most compelling reason to support the 'yes' campaign is that should any human be treated differently under the law?

"There's no doubt there are people with different religious views and faiths ...that doesn't stop any one of us from supporting what is right".

Ms Berejiklian was joined at the event by former NSW Governor Dame Marie Bashir, who actively supported groups such as gay and lesbian counselling services while in office.

"I'm extremely grateful to the gay community for what they have given in the elevation of this nation," Dame Marie said. "What a deeply felt privilege it is to support the promotion of marriage equality".

Energy and Arts Minister Don Harwin, who has been described as the most senior openly gay government minister in the nation, also spoke in favour of voting yes.

Other prominent Liberals in attendance included Christine Forster, who is a City of Sydney councillor and recently clashed with her brother, former prime minister Tony Abbott, who is strongly backing the 'no' vote.

Mr Greenwich told the event he was optimistic and that a poll released on Tuesday showed that a "good two-thirds" of the nearly 10 per cent of Australians who had cast their postal ballots had voted in favour of legalising same-sex.

Ballots for the Turnbull government's controversial $120 million postal plebiscite, which will not be binding on federal MPs, are expected to be delivered to all Australian households by September 25.

Same-sex couples have been able to adopt children in NSW since 2010 but previous attempts by a cross-party working group to generate state-based legalisation of same-sex marriage in the state have faltered in the face of legal hurdles.