The number of calls to Centrelink that have gone unanswered has increased by 61 per cent to 22 million calls a year, with bureaucrats saying smartphone dialling apps may be partly to blame.

Senate estimates has heard the number of "blocked" calls, when callers receive an engaged tone, is up from 13.7 million calls in the 2013-14 financial year.

Centrelink's chief executive Grant Tidswell said it was an issue the agency had been attempting to manage.

"Well, it's always a trade-off. I think we've talked about this at previous estimates about trying to manage the workload and demand and the challenge between demand and supply," he said.

He suggested one possible reason for the increase had been the use of smartphone applications, which can allow people to repeatedly call a number in a short period of time.

"Some people use it to get concert tickets and win prizes and we're getting people upwards of 800 to 1,000 times at that time," Mr Tidswell said.

"We are battling new technology with people doing that at peak demand times."

He told estimates there had been "well over 30 million" calls blocked in previous years.

Greens senator Rachel Siewert said the number was "staggering".

"To hear that 13.7 million has now swelled to 22 million makes my heart sink," she said.

"From questions during estimates it appears that Centrelink has no interest in taking further action in driving down the worrying statistic.

"At a time when the Government is happy to paint people on income support as 'dole bludgers', having infrastructure in place that clearly can't cope with demand shows a fundamental flaw.

"The Government should refocus its energy on fixing this burgeoning issue."

A total of 57 million calls were made to Centrelink in 2013-14, and 55 million calls between July 2014 and May 2015.