The Coalition government will pay contracted call centre operators for another 1500 staff to deal with surges in demand and cut its notorious phone queues.

It will add to 1000 private staff Human Services Minister Michael Keenan announced in April and 250 who began under a trial last year, bringing their number to 2750.

The expansion of contractors answering calls to Centrelink follows decisions to shed 2500 public servants in two years from the Department of Human Services overseeing the agency.

Mr Keenan said the call centre pilot, with contractors Serco, proved efficient and that its staff had helped reduce busy signals on Centrelink phone lines by almost 20 per cent.

The Department of Human Services has signed contracts with four private companies for the 1000 additional staff announced earlier this year to improve call wait times.

It will pay Serco for more staff at their Victorian call centres, while Stellar Asia Pacific, Concentrix Services and DataCom Connect will have contractors answer phones in Perth, Brisbane and Adelaide.

The main public sector union said privately employed call centre workers would not be the additional capacity that Centrelink needed.

Community and Public Sector Union national secretary Nadine Flood said the agency needed more well-trained permanent staff rather than contracted call centre staff.

"This costly process has taken months while there are empty desks in Centrelink offices around the country that could be filled almost immediately to provide the quality services that come with decent pay and comprehensive training," she said.

"The community expects to deal with professionals, not get shunted around private call centres staffed by people being paid thousands of dollars less, with massive staff turnover.

"Often these wasted hours on the phone only end when clients are transferred to a direct Centrelink employee who knows what they’re talking about."

Labor Human Services spokeswoman Linda Burney said the government was addicted to outsourcing and was privatising Centrelink piece-by-piece.

"Centrelink programs and income support are complex. That’s why it’s so important for Centrelink to have permanent staff who are trained and skilled to manage and assist the community," she said.

"There have been consistent reports of poor practices and standards at outsourced call centres. Labor is very worried about compromised service delivery, insecure working arrangements, lower wages, reduced conditions and poorer quality training."

Centrelink's phone services have been a source of massive frustration for clients left waiting for prolonged periods to reach staff or hitting busy signals, which met 55 million calls in 2016-17.

Clients report abandoning calls after long waits, and Human Services last year named hold wait times, engaged signals and call disconnections among the top complaints it received.

Department officials in May said average waiting time on calls had increased in the year to March 2018, from 15 minutes and 44 seconds up to 15 minutes and 56 seconds.

Mr Keenan said Centrelink answered about 1 million calls a week.