Crucial advice on the way to achieve Indigenous recognition in the constitution is likely to be delayed beyond the potential July double dissolution date.

Key points: Indigenous Referendum Council may miss June report deadline

Indigenous Referendum Council may miss June report deadline Report may not be tendered before election day if double dissolution is called in July

Report may not be tendered before election day if double dissolution is called in July Consultants still optimistic of May 2017 referendum date

Consultants still optimistic of May 2017 referendum date Patrick Dodson replacement expected to be announced in days

But one of the most senior consultants on the recognition process, Mark Leibler, said a referendum next May remains the "target".

The Federal Government — in consultation with Labor — appointed a Referendum Council at the end of last year, to "guide a national discussion" and consult on the content of the question.

The group, co-chaired by Mr Leibler, was expected to report back by the end of June, but Government sources told the ABC the council was unlikely to meet that deadline.

If the Federal Government makes good on its threat to call a double dissolution election on July 2 on the basis of blocked industrial relation bills, it would mean the report may not be complete before election day.

The potential for delay would further unsettle some advocates for constitutional change, who were already concerned the issue was fighting for public awareness in a crowded market, and would not be a priority during the election campaign

Voters will go to the polls in a general election this year. If the Coalition retains government, there will also be a national plebiscite on gay marriage.

A referendum could take voters back to polling booths for a third time in less than a 12 months.

Leibler 'cautiously optimistic' of May 2017 referendum

Mr Leibler said he still hoped to hand over a report from the Referendum Council by the June deadline.

Despite the lack of clarity about the detail of the referendum question, he said the "target" date for the poll remains next May.

That would coincide with the anniversary of the successful 1967 referendum, a landmark development in Indigenous rights.

"At this stage we're aiming for — the target is [May 27, 2017] — [the] 50th anniversary of the referendum and I'm cautiously optimistic that we can get there," Mr Leibler said.

Labor leader Bill Shorten has already committed to hold the referendum on that date.

The Prime Minister has not matched that promise, but has described a poll as "achievable" next year in the right circumstances.

"I think there's greater agreement across the various leaders on the Referendum Council, both non-Indigenous and Indigenous in terms of what is feasible and what might be doable. More so than in the past," Mr Leibler said.

"If we can pull this off it will send a message once and for all to the racists in our society that that sort of racism is completely and utterly unacceptable."

But National Congress of Australia's First Peoples co-chair Jackie Huggins has previously raised concerns about holding the vote on the anniversary date.

She said many Indigenous people did not support the timing.

"A lot of people are saying why have it on that date because it takes away from the historical impact and celebration of the time — especially if we get a no vote," Ms Huggins said.

"I mean, what is that going to do to that date? It might tarnish it forever."

Patrick Dodson replacement expected to be announced in days

The council has also lost a driving force, with co-chair Patrick Dodson stepping down to pursue a seat in the Senate for Labor.

Patrick Dodson has been nominated by Bill Shorten to fill Labor's Senate vacancy. ( AAP: Mick Tsikas )

A replacement for Mr Dodson and an additional committee member are expected to be appointed before the next meeting on March 21 or 22.

A spokesman for the Prime Minister said the "intention is to make the appointment as quickly as we can".

"We do not want the council to lose any momentum," Malcolm Turnbull's representative said.

Mr Leibler said he expected the replacement for Mr Dodson to be announced "within the next few days".

While he provided his own advice on the possible replacement, Mr Leibler would not be publicly drawn on his recommendations.

"I don't want to be placed in a position of pre-empting an appointment that has to be made by the Prime Minister and the Leader of the Opposition," he said.

The council has met a number of times, but is yet to begin formal consultations.

Mr Leibler said this month's meeting would finalise the consultation process.

"We want to reach agreement along those lines as soon as possible," he said.

The Victorian Government has begun discussions on a separate treaty process with Indigenous representatives.

It has been a decision that has caused frustration among some advocates for constitutional change, who fear the potential for the two issues to become confused.

"There's no problem whatsoever about state governments engaging with Aboriginal communities about entering into agreements, entering into treaties," Mr Leibler said.

"The issue of constitutional recognition is quite separate and distinct from that."