Tony Abbott has affirmed his commitment to delivering the national disability insurance scheme (NDIS) in a “sustainable, affordable” way as he spruiked an agreement to push ahead with trial sites in Western Australia.

The prime minister and the Western Australian premier, Colin Barnett, announced a deal to begin a trial site in the Perth Hills area on 1 July. The planned trials would benefit about 8400 people with disabilities in the state, the government said in a statement.

But Labor pointed to its own announcement of a deal with Barnett in August, a month before the 7 September election, when it flagged three trial sites expected to have 8380 participants. A government source said Monday’s agreement spelled out more details about the implementation of the pre-existing deal.

Abbott’s announcement came as leaders refined their pitches to voters ahead of Saturday’s Western Australian Senate election rerun.

It also followed tough talking from the treasurer, Joe Hockey, about the need to rein in spending ahead of his first budget in May.

Amid speculation about a possible delay in the full rollout of the NDIS, Abbott said the important thing was “getting it right rather than rushing it”.

A government statement said a trial site for the NDIS would be established in the Perth Hills area, alongside a trial site for Western Australia’s My Way scheme in the Lower South West region and Cockburn/Kwinana regions.

“The three trial sites will benefit just over 8000 people with significant disabilities,” Abbott said.

“The Commonwealth’s putting in about $80m, the state is putting in about $50m. This is yet another good example of how this government wants to work constructively and collegially with the states.”

Abbott said it was important to do the right thing by people with disabilities.

“Thanks to the agreements that the premier and I have signed today, we now have a national network of trials for the national disability insurance scheme and I do say very much ‘trials’ because I am happy to learn the lessons of these trials,” he said.

“I am happy to look at what works best, and obviously each state has its own slightly different way of doing things when it comes to delivering services to people with disabilities.”

Asked to rule out any delays to the full rollout, Abbott said the government would “absolutely deliver on all of the agreements that have been negotiated with the states and with the territories”.

“We are totally committed to delivering a sustainable, affordable national disability insurance scheme,” he said.

“We’re also listening to the NDIS board because we want to make sure that we get it right.

“In the end what’s important is getting it right rather than rushing it, and one of the problems with the former government was that they were rushing it because they wanted a political monument.

“They were more concerned with getting a political monument than they were with getting the right national disability insurance scheme.”

Labor has warned the government against walking away from its promises on the NDIS, and pointed to an announcement by the former Rudd government on 4 August about an agreement with Barnett to deliver the scheme.

The then minister for disability reform, Jenny Macklin, said the Commonwealth and WA government would jointly fund a launch site in the Perth Hills area, with about 4300 participants.

The two governments would also jointly fund two sites that will operate under Western Australian My Way model, in the Lower South West and Cockburn-Kwinana regions, with about 4080 participants, Macklin said at the time.

The opposition leader, Bill Shorten, also hit the campaign trail in Western Australia on Monday. He said he would not join “the Liberal chorus attacking the national disability insurance scheme”.

“The national disability insurance scheme is about empowering the lives of hundreds of thousands of Australians with disabilities and their carers who’ve been effectively exiled in their own country,” Shorten said.

“I am revolted by the fact that the Abbott government since they’ve got in has had nothing good to say about the national disability insurance scheme … The Abbott government is playing dangerously with the hopes of hundreds of thousands of people with profound or severe disabilities … I have no doubt that the Abbott government can't be trusted when it comes to standing up for people with disabilities and their carers.”

The Abbott government said the Perth trials would be independently evaluated by a joint steering committee of representatives from the federal and WA governments and community representatives from the trial sites.