The costs and sustainability of the National Disability Insurance Scheme will come under scrutiny after the Turnbull government announced the shape of a major review into its future.

Treasurer Scott Morrison said the Productivity Commission inquiry would help shape the final design of the $22 billion scheme - Australia's biggest social policy program since Medicare - which is due to be fully operational and available to nearly 500,000 people by 2019.

Under an original agreement between the federal and state governments the review was always scheduled to occur in 2017 but the release of its terms of reference come amid warnings the cost of the scheme could blow out by billions of dollars a year.

"This review is intended to inform the final design of the full scheme prior to its commencement by focussing on updating initial cost projections estimated by the Productivity Commission and by providing advice on longer-term projections and overall sustainability issues," Mr Morrison said in a joint statement with Social Services Minister Christian Porter released late Friday afternoon.