Prime Minister Malcolm Turnbull has cautiously responded to Indigenous leaders' calls for a constitutionally enshrined "voice" to parliament, warning that Australians are "conservative" about constitutional change.

Speaking at a lunch at Crown on Saturday to mark the 50th anniversary of the 1967 referendum, Mr Turnbull thanked Indigenous leaders who agreed on a historic declaration at Uluru on Friday to reject a minimalist version of constitutional recognition of Indigenous people. But he gave a thinly veiled warning that their more ambitious recommendation would face challenges in a nation in which referendums have historically been defeated.

Opposition Leader Bill Shorten, speaking at the same gathering, also did not specifically endorse the Uluru statement, but urged Australians to consider the calls with an "open mind".

The Prime Minister warned that a successful constitutional referendum must have "resolute solidarity" or "minimal or at least tepid opposition".

Mr Turnbull did not directly address the contents of the statement of the Heart, which outlined a roadmap for a treaty and called for the constitution to enshrine the Indigenous voice in parliament, through the creation of an elected indigenous advisory body.

"As I know better than most, changing the Australian constitution is not easy - 44 referendums and only eight successes," he told the audience, which included campaigners for the 1967 referendum and plaintiffs in the historic Mabo case.

"Indeed, history would indicate that in order to succeed, not only must there be overwhelming support but minimal or at least tepid opposition.

"No political deal, no cross-party compromise, no leader's handshake can deliver constitutional change. To do that, a constitutionally conservative nation must be persuaded that the proposed amendments respect the fundamental values of the constitution and will deliver precise changes that are clearly understood to be of benefit to all Australians."

The Referendum Council is yet to advise Mr Turnbull and Opposition Leader Mr Shorten in a report delivered on June 30.

Mr Shorten, who last year indicated a willingness to consider a treaty but has been more muted on the question since then, said there was "sincere desire for bipartisanship" on reconciliation.

"We owe those members and those that participated the time and the space to finish their work and we owe them an open mind on the big questions: the form that recognition takes on treaties, on changes required in the constitution and on the best way to fulfil a legitimate and long-held position for Aboriginal and Torres Strait Island people," he said.

But he warned that arriving at a final proposal may be challenging.

"I do not doubt the size of the mountain that we will have to climb," he said. "But for any Australian in need of inspiration, I would say look to our history, look to that spirit of '67, or Eddie Mabo."

Greens leader Richard Di Natale criticised the Prime Minister for not endorsing the Uluru declaration. He supported calls for a treaty and an Indigenous voice in Parliament.

"I'm deeply concerned, the Prime Minister had an opportunity today to say, 'I stand with our First Nation people, I've heard them and we are going to work towards a treaty and towards as a strong Aboriginal voice', and instead he appears to have backed away from any significant change," he said.

The National Reconciliation Week Luncheon marked the first opportunity for both leaders to respond to the Uluru Statement, which moved away from the symbolic recognition of Aboriginal and Torres Strait Islander people in the constitution in favour of enshrining a "First Nations Voice".

"In 1967 we were counted, in 2017 we seek to be heard," read the statement.

The Uluru declaration also called for a "Makarrata Commission" that would supervise agreements between Indigenous groups and government and a period of "truth-telling" about the history of Aboriginal and Torres Strait Islander people.

"Makarrata is the culmination of our agenda: the coming together after a struggle," read the statement. Makarrata is a Yolngu word for treaty or settlement.

The Referendum Council's co-chair Pat Anderson said in its statement on Friday that "delegates agreed that sovereignty has never been ceded or extinguished".

High rates of suicide, closures of communities and youth detention were all proof of the need for constitutional reform, the co-chair said.

The 1967 referendum included Aboriginal Australians in the census. The luncheon also fell on the 25th anniversary of the High Court's decision to uphold native title rights in the Mabo case.