High Court to give reasons for same-sex marriage ruling.

The High Court will today make public the reasons for its decision to allow the same-sex marriage postal survey to go ahead.

It comes three weeks after it found the finance minister did have the power to fund the $122 million voluntary survey.

Two groups of same-sex marriage advocates tried to stop the postal survey, arguing the government should not have bypassed parliament in funding it.

The voluntary survey was the government's plan B after the Senate blocked the compulsory plebiscite promised by the coalition at the 2016 election.

The survey closes on November 7, with results to be published on November 15.