The finding by the Commonwealth Ombudsman has prompted 20 recommendations for the agency overseeing the NDIS.

The agency admitted to the ombudsman in February it had more than 8000 reviews on hand and was receiving around 620 new review requests each week.

"The NDIS has acknowledged some reviews are taking up to nine months to be completed," the ombudsman said in the report, released on Tuesday.

A new national team set up in November 2017 to address the backlog has not been able to quickly work through the wait list.

The ombudsman's office received 400 complaints between July 2016 and January 2018 that were, at least in part, about reviews.

"Complaints to our office also highlight other systemic issues such as poor communication about review processes and outcomes," the ombudsman said.

Those who complained said when they requested a review they didn't receive an acknowledgement, so they were unsure whether it was going to happen.

Others said when they tried to get an update on their review, their calls were not returned.

The ombudsman wants the NDIS agency to set up standard operating procedures so staff acknowledge review requests in a set time frame.

The scheme's agency has accepted all the recommendations and the ombudsman will monitor how it implements them over coming months.

Australian Associated Press