Scott Morrison expects to secure terms of reference and state agreement for a royal commission into the disability sector before the federal election in May.
The prime minister predicts state and territory governments will come on board "within days", saying it is imperative they are involved.
"To not have those jurisdictions subject to the royal commission, I think, would impair it overwhelmingly," he told reporters in Tasmania on Wednesday.
The prime minister does not technically need state support to launch the inquiry, but does want them to share the costs and responsibility.
Opposition Leader Bill Shorten has pledged $26 million to establish a disability royal commission.
Mr Morrison expects the costs to be much greater.
"I think it will be a royal commission of a similar size and standing as what we saw with institutional child sexual abuse," he said.
"I don't know what sort of royal commission he (Mr Shorten) was talking about but it clearly was not one that he'd properly thought through."
The child abuse inquiry cost $372 million over five years.
Mr Morrison last week wrote to state and territory leaders seeking their support.
NSW, Victoria, South Australia, Tasmania and Queensland have told the Commonwealth they will back an inquiry.
Western Australia has offered in-principle support.
"However, due to the state's budget situation, WA has not agreed to cost-sharing arrangements," a spokesman for the premier told AAP.
Labor has accused the prime minister of haggling with the states and territories to "defer and delay" the royal commission.
"This is a cheapskate move," deputy leader Tanya Plibersek said.
"This royal commission should be fully federally funded and Scott Morrison could easily set aside the money for this in the budget."
The coalition government backed a motion supporting the royal commission in the House of Representatives last week under pressure from Labor and the Greens.
It passed the Senate four days earlier without coalition support, after being raised by Greens senator Jordon Steele-John.
He wrote to state and territory leaders a day before Mr Morrison, personally asking for in-principle support, and said on Twitter that six of the eight premiers and chief ministers had given it.
