Malcolm Turnbull is facing a fresh outbreak of internal dissent over the proposal to recognise Indigenous Australians in the constitution before talks about the referendum on Thursday with the Labor leader, Bill Shorten.

The South Australian Liberal senator Cory Bernardi told Guardian Australia on Wednesday “no case had been made” for recognising Indigenous people in the constitution.

Bernardi said years of discussion about recognition had failed to yield a concrete proposal to put to a referendum and as he was a “constitutional conservative ... it’s highly unlikely I’d agree”.

Fellow Liberal senator James Paterson told Sky News on Wednesday he was yet to be convinced constitutional change was the appropriate way to proceed.

“There is no place for race in our constitution,” Paterson said. “There should be no negative references to race, there should be no positive references to race. [The constitution is] the rule book of Australia. I think there is a role for symbolism in public life but I’m yet to be convinced the constitution is the place for that.”

The new bout of restiveness has been triggered by a proposal in Western Australia. The WA branch of the Liberal party’s youth wing wants to bring forward a motion at the state conference next week calling on the federal government to oppose recognition and campaign against constitutional change.

Bernardi told Guardian Australia a binding motion such as the one envisaged in WA would go against the culture of the Liberal party but, with that said, he was not, intrinsically, a supporter of the constitutional recognition proposal.

The prime minister will meet the Labor leader on Thursday to discuss both constitutional recognition and also the marriage equality plebiscite.

Shorten has been flagging a treaty as the next logical step after recognition and a number of Indigenous leaders are no longer interested in the incremental step of recognition.

At the Garma festival over the weekend, Noel Pearson, who is on the referendum council, called for a synthesising of the treaty and constitutional reform arguments.

“If we think they are somehow separate agendas, this whole agenda will fail,” Pearson said. “My synthesis is simply that constitutional recognition provides the hook that enables agreements to be made, and a Makarratta, a national settlement, to be made.”

The Labor senator Pat Dodson, who was a co-chair of the referendum council until he decided to stand as a Labor senator in the recent election, also spoke of a post-recognition settlement. “If there is no preparedness to do that, we are all wasting our time and the tax payers have been dudded,” Dodson said.

On Wednesday the prime minister was asked whether he was worried that the middle ground on this debate was being lost, given members of the government were digging in their heels, and Labor, with the backing of some Indigenous leaders, was already pitching about the next stage.

Turnbull told reporters it was “very hard” to change the constitution. He said he “earnestly” sought to achieve constitutional recognition.

“But there are some important steps that need to be achieved,” the prime minister said.

“Firstly, there needs to be agreement coming from the [referendum] council as to what language they would propose to put into the constitution and then we have to be satisfied, as I believe as all of us are in the parliament, people of goodwill be satisfied, that that language is language that meets the purpose, and of course is capable of winning support in the referendum.

“It is a very, very difficult business. Changing our constitution has proved very challenging and we shouldn’t underestimate the difficulty but we are committed to doing it, but we have to make the first step is to see the proposed language, proposed amendments from the council.”

The referendum council meets again next week.