One of the federal government's largest departments fears industrial action will hurt Centrelink, Medicare and child support customers in the lead up to Easter.
The Community and Public Sector Union is planning a full day work stoppage at the Department of Human Services on Monday.
DHS has one of the highest rates of union membership in the Australian public service and two offers have already been crushed by a large majority of staff.
DHS spokesman Hank Jongen said even contingency arrangements would probably not avoid disruption and delays for customers.
"The lead up to Easter is always an extra busy time because our staff are already working hard in a short week to ensure people are paid before the public holidays," Mr Jongen said.
"This action further stretches our resources and punishes people who need government support.
"We are concerned the union is encouraging its members to take counterproductive industrial action, which will inconvenience people across Australia, including some of the most vulnerable people in our community.
"We are confident customers' payments will occur as usual and we will do our best to minimise disruptions to services.
"However, we may have reduced numbers of staff in service centres and on the phone and increased wait times on Monday."
Customers are being advised to delay contacting DHS about non-urgent issues or to use self-service options.
Included in its list of demands the CPSU has called for greater consultation, protection from the casualisation of the workforce, split shifts and the changes of rosters at short notice and the right to job swap during the redundancy process.
The union also wanted a pay increase of 2.5 per cent to 3 per cent a year plus recognition of the financial disadvantage caused by the delay in settlement calculated at 3 per cent for two years. The current agreement expired almost two years ago.
The previous offer, rejected by about 80 per cent of staff who voted, included average pay increases of 2 per cent a year for three years.
CPSU national secretary Nadine Flood said "it's a bit rich for DHS bosses to say the strike would inconvenience families when they are trying to strip family-friendly working conditions from their own staff".
"We have working women in Centrelink and Medicare telling us they'd have to give up their jobs at DHS if this agreement went through, with proposals like workers being forced to work anywhere in a major city, different hours and days of work or even split shifts," she said.
"The reality is that these workers have been fighting for two years to get a fair deal where they can keep those important rights and that's why they're striking.
"Our union has put out a sensible, reasonable outcomes position with workers willing to agree sensible changes to agreements, including in DHS, but the Turnbull government is still refusing to move on key issues like stripping essential family-friendly rights and conditions out of agreements, preventing any improvements like domestic violence leave and no recognition of the impact of a two-year wage freeze."
There will also be all-day strikes on Monday by staff at the Australian Tax Office, Defence Department, the Bureau of Meteorology, the Department of Prime Minister and Cabinet and the Australian Bureau of Statistics.
The union said weeks of rolling strikes at international airports during the Easter school holiday and beyond, including a 24-hour strike across the Department of Immigration and Border Protection and the Department of Agriculture and Water Resources, would follow.
The strikes come after a majority of staff at the Australian Competition and Consumer Watchdog rejected a proposed pay deal on Thursday.
Community and Public Service Union president Alistair Waters said 77 per cent of staff voted on the agreement, with close to 80 per cent rejecting the deal.
"There's growing frustration across the APS because workers have been fighting for two years to get a fair deal, in the face of a pay freeze and unfair attacks by agencies as is required by the Government's bargaining policy," he said.
"More than eight out of 10 public sector workers still don't have new agreements."