Some routine case reviews are being kept waiting for nine months while Centrelink's parent department has axed 800 call centre jobs, with another 400 to go before the end of the year.

The Department of Human Services refuses to say how many case reviews are outstanding but insiders say the list blew out to more than 24,000 earlier this year, forcing the department to divert resources to call claimants and try to convince them their review was unnecessary.

The push was partially successful with 2500 cases taken off the books but Fairfax Media understands that about 1000 new review requests piled up at the same time.

Welfare advocates say that six month waits for reviews are “commonplace” and a major pressure point is the Disability Support Pension, with tougher eligibility rules leading to more refusals and a greater number of review requests.

But departmental sources say that it is the increasing complexity of Australia's welfare system and the bewildering array of payments administered through Centrelink that is a straining the system to breaking point.

One bright result for DHS this year is the number of complaints to the Commonwealth Ombudsman has fallen, with 5093 Centrelink complaints in the 2012-2013 financial year compared to 6133 in the wake of a withering critique in 2012 by the watchdog of the agency's review system.

A spokeswoman for the Ombudsman's office said the reduction was attributed to a new telephone queuing system that refers complaints straight back to Centrelink in the first instance.

A DHS spokesman said Centrelink's average time to answer a phone call in the first months of the 2013-2014 financial year was 12 minutes against its “key performance indicator” of 16 minutes.

But the traditional peak demand season of January to March is expected to blow those waiting times out to 16 minutes and more.

In a statement, the spokesman defended his agency's performance in finalising reviews.

“In August, around two thirds of reviews were finalised within 35 days which is comparable with normal activity,” he said.

“We also prioritise reviews for vulnerable customers and over 90 per cent of these are finalised within fourteen days.”

Centrelink also said that the vast majority of cases are handled without the need for a review.

“In making payments totalling more than $144.7 billion on behalf of government, the Department of Human Services makes millions of decisions on an annual basis relating to Centrelink entitlements, and only a small number of these decisions are reviewed,” the spokesman said.

But the National Welfare Rights Network president Maree O'Halloran said her organisation held little hope of Centrelink meeting its customer service targets without making major changes.

“The Centrelink review and appeals system is under considerable strain,” Ms O'Halloran said.

“The Department of Human Services is taking steps to address these delays, but we do not expect the Department to meet its performance indicators from its customer Charter anytime soon.”

Ms O'Halloran said the “unacceptable” backlog in reviews was leading to “review fatigue”, where the claimant simply gives up.

“Without a doubt, excessive delays are undermining people's right to have questionable Centrelink decisions reviewed,” she said.

“It is not uncommon to hear of delays of six months, or more.

“This is an unacceptable situation.

“The huge volume of appeals waiting to be finalised raises questions about the impact of possible job cuts in the Department of Human Services.

“There is a clear need to boost staffing numbers at Centrelink to cope with the backlog.”