Marriage equality is four votes from winning a vote in parliament but some major hurdles still put the reform out of reach.

Sixteen Coalition MPs in the lower house have privately told Australian Marriage Equality they support the gay marriage reform, along with five who have publicly supported it.

If the current numbers were to count the Coalition would need to be given a free vote, which the party has so far not allowed.

• Australia tells Tony Abbott to ‘wait in the car’
• PM snub story a ‘disgraceful smear’: Lib MP
• MPs switch sides on same sex marriage

Former deputy prime minister Wayne Swan has shifted to support the reform, and a poll by AME has suggested they are four votes away from a majority in the lower house if a free vote was allowed.

Marriage equality campaigner Rodney Croome was a 2015 Australian of the Year finalist. Photo: AAP

But there are still plenty of MPs in the opposition who would oppose gay unions, and Australian Marriage Equality national director Rodney Croome confirmed there was a pattern to their support.

“Generally, those who are backed by the (Shops, Distributive and Allied Employees Union) SDA don’t support it,” he said. “That seems to be the main factor at the moment.”

He said some SDA-backed MPs and Senators have said they will back the reform, but most do not.

Among the Coalition he said the religious affiliation and their age was “probably a better guide to where they stand on the reform”.

In the lower house a free vote on the issue would garner 72 votes, short by four of the 76 needed to win.

AME bases its numbers on those MPs that have publicly declared their support and those who have privately declared it to the lobby group.

In the Senate the Greens and Labor hand 26 votes to supporting the reform and among the Coalition five MPs have publicly supported the reform and eight privately.

Mr Croome said any vote on the issue would be premature, as Liberal Democrat Senator David Leyonhjelm’s Freedom to Marry Bill is before the parliament at the moment.

“I’d welcome further debate on his bill, but I would certainly advise against bringing it to a vote because we don’t have a coalition free vote yet,” he said.

“If it does (come to a vote), that bill will be defeated again.”