The architect of the national disability insurance scheme has rejected suggestions the landmark program is inefficient.

But the Abbott government is sending clear signals the NDIS may not be rolled out in the same form or follow the timetable laid out by Labor.

Treasurer Joe Hockey says a “massive blowout” in the scheme’s initial costs shows Labor “got it all wrong” in the rollout of trial sites five months ago.

The government has said it is in discussions with the states and territories to ensure the scheme’s efficiency and cost effectiveness.

“The bottom line … is we have to find ways to deliver services within an existing budget framework rather than on the never-never,” Mr Hockey told ABC radio on Wednesday.

The treasurer said the government would ensure “a proper rollout” of the NDIS, indicating there may be changes to its design.

Opposition families spokeswoman Jenny Macklin, who introduced the scheme when Labor was in government, is demanding the coalition rule out capping support or slowing the rollout.

“There is no evidence whatsoever that it is ineffective or inefficient,” she said.

Initial data from the scheme’s first three months shows demand is higher than expected and the cost is running 30 per cent over estimates.

More trial sites will be launched next year with expectations the scheme will be fully rolled out by 2019.

The annual cost of the NDIS, when fully operational, is estimated at $22 billion.

Finance Minister Mathias Cormann said the government remained committed to the NDIS, but it was important funding was “better targeted” so the scheme was as cost effective as possible.

Shadow treasurer Chris Bowen says that language hints at the government’s hidden agenda.

“That sounds like code for cuts to me,” he told ABC television.

But Assistant Treasurer Arthur Sinodinos said the government would not “cut across” any election commitments, and that included rolling out the NDIS.

The government would carefully go through the evidence from the NDIS trial sites to ensure the scheme was delivered in a “value-for-money way”.

“We’re seeking to make sure that we deliver a scheme which is as cost effective as possible, while meeting the needs of the constituency that we’ve identified,” he told ABC Radio.

“But we won’t be able to look after them if we design a scheme that is not fiscally sustainable.”

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