Corporate Australia is mobilising to join a fierce campaign over same-sex marriage which will be slugged out by third parties, including churches, unions and activist groups over the next three months.

But not everyone is on board yet, with Qantas chief executive officer Alan Joyce slamming the Coalition's decision to settle the matter by postal vote or plebiscite as a waste of money. While there was still a prospect of the postal vote being stymied by a court challenge, Mr Joyce said Qantas would reserve its position on whether to join the fray.