Australian Marriage Equality (AME) said their analysis finds at least 76 out of 150 MPs in the House of Representatives and 41 people in the 74-seat Senate support a broader definition of marriage. He added that a number have also indicated they would support a change, if given a free vote.
The list includes 72 MPs who are on the record as supportive of change, while six more have indicated confidentially that they are now more inclined to also back same-sex marriage.
Australian Marriage Equality national director Rodney Croome said the $160 million cost of holding a plebsicite would be a waste of money.
"The more Coalition members I speak to about this issue, more they worry whether we actually do need a plebiscite," Australian Marriage Equality national director Rodney Croome told ABC News 24.
"It would also be quite divisive through the community and it will mean the issue continues to be a live issue right up to the election and beyond when they want to focus on issues like the economy and national security."
Mr Croome said the recent debate about whether Coalition MPs will respect the outcome has brought "simmering tensions" to the surface.
"Given that mess, surely the easiest thing to do is bring it back to parliament, have a vote, get it done and move on," he said.
"My advice to Malcolm Turnbull is if you pull off the band-aid now, most people would be pleased with that.
"It will be over in a month and you can move on to the election without this being a distraction."
Some conservative MPs and senators, including dumped cabinet minister Eric Abetz, have flagged their right to a conscience vote in parliament, potentially ignoring the will of voters.
Former prime minister Tony Abbott said he would support the result of a plebiscite on same-sex marriage.
Fresh from addressing a conservative US lobby group, Mr Abbott says parliament should respect the people's decision, whatever it is.
The Australian Christian Lobby, which argues against changing traditional marriage, says the $160 million price tag of the promised plebiscite is reasonable to settle "a long running campaign" by activists.
"They keep claiming overwhelming support yet don't want to take the issue to the people. This doesn't make sense," ACL managing director Lyle Shelton said in a statement.
If the government is returned at the next election MPs who support same sex marriage will be asked to finalise a bill, which will then be put to the people at a plebiscite.
Mr Abbott's successor Malcolm Turnbull said on Friday if the majority of people voting in a plebiscite vote in favour of it, then same-sex marriage will be legalised.
