More than 55 million calls to Centrelink were met with a busy signal in one year, a figure labelled "astronomical" by a Greens senator.

The number of busy calls during the 2015-2016 financial year was nearly double the number a year before, a Senate committee heard on Wednesday night.

"That is a lot of frustrated people who may be exasperated and struggling," Greens senator Rachel Siewert told Department of Human Services officials during a estimates hearing, adding the number was "astronomical".

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But departmental official Renee Leon said the increase was down to automatic dialling apps.

"That's not 55 million unique calls," she told senators, adding analysis of the telephone data showed that about 20 per cent of the calls were from repeat dialling apps.

"55 million doesn't mean there were 55 million people who tried to ring, because many of those were repeat calls."

Labor says Centrelink is under-resourced and understaffed.

"Centrelink needs permanent, full-time staff, who are qualified and familiar with the often complex issues facing Australians who require income support," Opposition social services spokeswoman Jenny Macklin said in a statement.