The figures, from June 2016 to April 2017, are a significant turn for the worse on the previous 12 months when 29 million calls received the dreaded beep-beep-beep.

In 2014-2015, the number was 22 million.

Centrelink's parent department, the giant Human Services, told a Senate Estimates hearing in Canberra late on Thursday night that many callers were using mobile phone apps that allow them to redial "every couple of seconds".

Now, the department was trying to figure out how widespread was the use of the redial apps.

"The analysis we've asked our provider to do is to strip out those that try less than a number of times per segment of time, because obviously it's reasonable for them within a minute or two to try again, but it's actually not reasonable for them to try a thousand times a day, you know," chief information officer Gary Sterrenberg said.

The department's boss Kathryn Campbell told the committee that DHS was hiring a private firm to provide 250 call centre operators to answer phones, but that they would be mostly taking the "simpler" inquiries.

"Hopefully if they were ringing, for example, 20 times a day, if we're able to take them on the first

But the main public service union, the CPSU, says the 250 private sector telephone operators, which were announced in May's Budget, pale beside the 1180 jobs that are set to be axed from the department.

"Anyone who's tried to get help with a Centrelink, Medicare or Child Support query, whether it's over the phone or at a walk-in centre, knows that it's never been harder to get the help they need," the unions national secretary Nadine Flood said on Friday.

"It shouldn't be this hard for Australians to access essential public services."

"They've slashed 5,000 permanent jobs out of DHS in recent years and they've kept cutting and still are cutting even as these unanswered call figures have grown and grown and grown.

"The Government tried to pay lip service to the problem in last month's Budget by promising 250 outsourced call centre workers, but at the same time announced nearly another 1,200 DHS jobs would be cut."

The Greens too said they were skeptical of the ability of the private sector deal to turn Centrelink's worsening customer services performance around.

"Forty-two million busy signals, this is an astronomical number representing Australians trying to access supports, adjust their payments, seek information or update their earnings," West Australian Greens Senator Rachel Siewert said.

"That is a lot of frustrated people who may be exasperated and struggling.

"We have a system that is so broken that every year the busy signal wait times just climb and climb. Last year the blocked calls was nearly 29 million, the year before it was 22 million.

"When is this exponential increase going to stop?

"I'm not convinced that the Government's proposed piloting of Government service provider run call centres will solve this issue."