Labor candidate Evan Hughes says he will challenge Malcolm Turnbull for the Sydney seat of Wentworth because the prime minister has abandoned causes he once championed.

"I will defend the Malcolm of old. I certainly defend the Malcolm that Wentworth fell in love with, that Malcolm that believes in same-sex marriage and action on climate change and all those sorts of things, the Malcolm we all used to know," Mr Hughes told the ABS's 7.30 program.

The gallery owner said same-sex marriage and climate change were important to all Australians but Mr Turnbull had let down those who hoped he would make progress on the issues as PM.