Australians with a disability might have to wait a little bit longer for the complete roll-out of a landmark scheme designed to cover their care and needs, if the Abbott government accepts advice from its commission of audit.

A day before Prime Minister Tony Abbott meets with state and territory leaders to discuss progress of the national disability insurance scheme, the commission warns the implementation timetable is highly ambitious.

The NDIS is scheduled to be fully operational in 2018-2019, covering about 450,000 people with disabilities.

The commission acknowledges the worthiness of the scheme and the fact that it enjoys widespread community support and bipartisan political backing.

But it’s concerned about early cost blow-outs at the four trial sites that began in July 2013, noting average package costs of $46,000 per year exceeded the $35,000 figure factored into the scheme’s initial design and costing.

The NDIS Agency on Thursday insisted it was operating within its budget and had the capacity to meet the roll-out schedule.

The commission is not so sure, saying there was a risk of inadequate delivery of services to participants as well as significant financial risks to the scheme.

It recommends there is merit in pursuing a slower roll-out schedule.

It also wants government to take control of the scheme’s budget, which it estimates will exceed $25 billion after 2019, arguing such an amount was too great a responsibility for an independent board.

The commission also questions whether the assessment of potential participants should be outsourced as it is now.