The referendum council overseeing plans for the vote said on Tuesday it would continue its discussions with Indigenous and non-Indigenous Australians into next year, with plans to provide its final report to Prime Minister Malcolm Turnbull and Opposition Leader Bill Shorten later than expected.

The decision makes a 2017 vote, originally proposed by former prime minister Tony Abbott, highly unlikely and also removes the possibility that Australians could be required to take part in the same-sex marriage plebiscite and referendum in the same year.

The right-wing think-tank, the Institute of Public Affairs, called for the government to end the process, arguing the new delay meant public support for change was unfeasible.

Council co-chairs Mark Leibler and Pat Anderson said Tuesday's meeting in Melbourne agreed an interim report on the process would be handed to Mr Turnbull and Mr Shorten by September 8 this year, with an outline of progress so far and future steps for consultation.

In a statement, the pair said the council had noted the strong message from participants that the consultation process should not be rushed "by working to an artificial deadline", to meet the 2017 anniversary.

A final report would be completed by mid-2017.

Mr Leibler told ABC radio on Tuesday the concept of a treaty was "on the table".

"What we may well end up saying is 'here is the proposal for amending the constitution through a referendum' but we think if this is going to be successful, if you want to guarantee its success... you need to understand there are other issues around, treaty may be one of them, which have to be dealt with at the same time.

He said the preferred timing of Indigenous leaders needed to be respected.

"It actually would have been very nice to do it on that date, there's is great symbolism attached to that date," he said.

"What is much more important than the symbolism attached to a date is to make sure we carry out the consultations properly and the Indigenous leaders feel they need more time to get this done and to get it done properly."

Last week, Indigenous Affairs Minister Nigel Scullion said Parliament should consider a treaty with Indigenous Australians if it is recommended by the council.

The comments, at odds with statements by Mr Turnbull during the election campaign, come as some participants in the council's consultation have argued for the constitutional recognition process could be a wasted opportunity, and efforts should instead go towards negotiations for a treaty.

Before the July election, Mr Turnbull criticised Mr Shorten over his willingness to consider a treaty agreement similar to those in place in New Zealand and Canada, arguing it could jeopardise efforts for securing a consensus.

Mr Shorten considered a treaty as a second step, after changes to the constitution.

Activist and lawyer Michael Mansell and Victoria University professor Gary Foley are among Indigenous leaders pushing for a treaty, while chair of the prime minister's Indigenous Advisory Council, Warren Mundine, has argued recognition and moves for a treaty are not necessarily mutually exclusive.

IPA policy director Simon Breheny said nine years had passed without a referendum proposal emerging.

"The decision of the referendum council to extend their consultation is further evidence that none of the suggested proposals for constitutional change are feasible.

"Australians will reject any proposal to insert race into the Australian constitution because such a proposal is divisive and dangerous," he said.

"The government should take steps to move this issue off the agenda."

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