Openly gay Labor MP Penny Sharpe has defended party leader Luke Foley's indecision on the issue of same-sex marriage, even as his stance threatens to hurt her chances in the progressive seat of Newtown.

Ms Sharpe, an upper house MP and strong same-sex marriage advocate, is seeking election to the new inner-city seat in March. Newtown has a higher-than-average proportion of same-sex couples and is notionally a Greens seat on current voting patterns.

Penny Sharpe has defended party leader Luke Foley's indecision on the issue of same-sex marriage. Credit:Wolter Peeters

Mr Foley voted against same-sex marriage in an upper house conscience vote in November 2013, and the bill was narrowly defeated.

Since assuming the leadership on Monday Mr Foley, a committed Catholic from the party's left faction, has been criticised on social media and elsewhere for failing to endorse gay marriage, a stance at odds with NSW Labor policy.