Shadow Attorney-General Julian Leeser and Liberal leader Peter Dutton say they do not believe the proposed Voice to Parliament is "on track for success" following a meeting with the referendum working group.
Mr Leeser and Mr Dutton met on Thursday with the working group to discuss their concerns over a lack of detail and "lack of comfort" with the wording of the proposed constitutional amendment.
"The wording does keep changing, it started off with three sentences, then it became four sentences," Mr Leeser said.
"It goes back to something we have been saying and we reiterated in the meeting, there's a lack of proper process around all of this."
He repeated calls for the government to provide a formal response to a report proposing options for how the Voice to Parliament may work.
Debate over proposed referendum wording
The words "executive government" are proving a sticking point in the proposed constitutional amendment to recognise and create a Voice for Aboriginal and Torres Strait Islander Australians.
The referendum working group, the government’s main advisory body, met on Thursday to consider what advice it would give to the federal government on the wording of the question and constitutional amendment for a referendum.
One expert lawyer advising the committee has recommended removing the wording "executive government" altogether, and is understood to have said if those words remain the referendum will be lost.
The referendum group, made up of more than 20 Indigenous leaders and legal experts, rejected the advice to remove "executive government".
They argued it was crucial the Voice have the power to advise government at the highest level, including advising bureaucrats working on laws and policies.
It is understood the group agreed that removing "executive government" from the proposed wording would significantly weaken the power and scope of the Voice.
But, outside of the meeting, a senior government source has told the ABC there could be extra words added after executive government to include "of the Commonwealth".
It is unclear what the referendum working group's position is on including additional words in the amendment.
There are only two meetings before the wording must be finalised to be put to the parliament in the constitutional amendment bill.
The debate over the wording highlights the extent to which granular details are being considered.
Sources have also told the ABC there was concern expressed by one person in the meeting that the current wording could leave the government open to challenges in the High Court.
The working group is also considering the official title of the proposed body.
The working group debated whether the body should be called the "First Peoples Voice" or "Aboriginal and Torres Strait Islander Voice", or "First Nations Voice".
Seemingly in response to Opposition Leader Peter Dutton's call for more detail, sources have told the ABC there was a major discussion about how much detail should be released, with one attendee telling the ABC "there is going to be more detail", adding, there is a "sweet spot" between being informed and being confused.
The current draft wording is:
In recognition of Aboriginal and Torres Strait Islander people as the First Peoples of Australia:
1. There shall be a body, to be called the Aboriginal and Torres Strait Islander Voice.
2. The Aboriginal and Torres Strait Islander Voice may make representations to parliament and the executive government on matters relating to Aboriginal and Torres Strait Islander peoples.
3. The parliament shall, subject to this Constitution, have power to make laws with respect to the composition, functions, powers and procedures of the Aboriginal and Torres Strait Islander Voice.
The constitutional amendment bill is expected to be tabled in parliament next month.
