Public servants working in the Child Support agency were left grappling with a meltdown that made the systems completely inoperative beginning on Thursday last week and ending by Wednesday, according to the Community and Public Sector Union.

The Department of Human Services denied the Child Support system was completely inoperative, saying it had been fully restored and the problems resolved after some customers and staff had experienced "intermittent issues".

While the CPSU said it feared some families may not receive payments before the Easter long weekend, the DHS said Child Support customers had not missed payments that were due, and would not be affected before the holiday.

The union said only limited systems came back online on Wednesday morning and Child Support public servants reported they remained unstable and sluggish.

The tech failure is the latest in a series of IT problems across the Australian Public Service and another for the Department of Human Services, which saw its long-awaited new payment system for Australia's $3 billion child support program run into trouble almost immediately after its launch last year.

The CPSU said the computer systems used by Child Support staff were inoperative for four business days, leaving the agency's public servants taking clients' numbers and promising to call back when the tech problems were resolved.

"Even now staff are facing a massive backlog and a sluggish and unstable system that's a long way from working as it should," deputy national president Lisa Newman said.

"This major system failure is an absolute disaster for people who rely on Child Support payments or other services and assistance.

"What's worse is that the department appears to be leaving staff largely unsupported as they've dealt with frustrated customers and rising levels of aggression."

DHS spokesman Hank Jongen said its management had supported staff with regular updates during the tech failures.

Labor digital economy spokesman Ed Husic said there had previously been questions about the Pluto system, and the department should make clear what happened in the latest tech failure.

"It's clear the DHS has not ruled out that something happened.

"Given it recently said that the robo-debt debacle was something that went well for them, are they suggesting these outages were something to improve online access?

"It just beggars belief. Spin won't cut it, facts will."

Both frontline public servants and the CPSU told Fairfax Media last year that large numbers of Child Support staff were being ordered to drop everything to help with "emergency escalations" as the agency's phone lines were swamped with irate clients following its adoption of its new Pluto system.

Human Services said then the roll-out of the new system had been going smoothly, and denied that any "emergency escalations" occurred during the "implementation" period.

Child Support staff reported the new system was slower and clumsier than the obsolete technology it was supposed to replace.

Ms Newman said DHS' Pluto project had been heavily reliant on external contractors rather than in-house expertise, leading to delays and cost blow-outs.

"This process needs to be brought back in-house urgently to ensure there's the integrity and accountability that's needed to get things running properly," she said.