Only 40 million of the 62 million attempts to contact Centrelink by phone in 2014-15 were successful, meaning the agency answered 4 million fewer calls than it managed the previous year.

The proportion of calls being answered has plummeted from 75 per cent in 2013-14 to 64 per cent in 2014-15.

The deteriorating performance comes despite the agency pledging to do more to improve its customer service effort after it was savaged in a mid-2015 report by the Australian National Audit Office, answers to Senate inquiry questions reveal.

Australians spent 143 years waiting in vain to speak to Centrelink in 2013-14, before simply hanging up, the auditors calculated.

Centrelink's key strategy in combating the problems with its telephone service has been to try to divert clients onto its online services, but the system was dogged by mass lock-outs, meltdowns and other glitches during 2015.

Labor's Human Service spokesman Doug Cameron says he is "appalled" by the latest customer service figures from Centrelink's giant parent department Human Services.

"I'm appalled by the diminution of service to Australian citizens," the Senator said.

"Year on year, the customer satisfaction [with Centrelink] declines, service delivery declines, the number of calls answered declines – this really is a department and a minister that don't seem to be capable of delivering the service to the customers."

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Senator Cameron said that staff morale at DHS was low, with the long-running wage dispute at the giant department a long way from resolution.

"The Australian National Audit Office said in a report that is now more than a year old, that when you move to electronic delivery, it creates its own demand for face-to-face and personal delivery, especially given the complexity of the work carried out by Human Services," Senator Cameron said.

"So there should be a range of accessible channels for service delivery from government and while we should continue to look at electronic service delivery, there has to be a realisation that on its own that will not solve the problem."

In its defence, Human Services told the Senate committee that a large number of emergency calls in 2014-15 had made its work more difficult that year.

"A contributing factor to the reduction in successful calls in 2014-15 was the diversion of staff to events such as emergencies," the department wrote.

"When compared to 2013-14, there was a significant increase in the number of emergency calls and also the time to handle these calls.

"In addition, there has been an increase in the complexity of calls, with the average handle time on four of the five main business lines increasing."

The department also says it has gone on a recruitment drive for hundreds more public servants to answer its phones.

DHS continues to put its faith in online service delivery, pointing to consistent and strong growth in the take-up of its apps and the use of its websites by clients to conduct their business.

"New technologies and practices are delivering genuine improvements in service delivery," the department wrote to the senators.

"This is consistent with the aim to allow people to do their business at a time that suits them and in a way that is consistent with how they use other services in the community."