Beleaguered Centrelink staff are bracing for a "perfect storm" of customer service problems in the coming weeks, heaping more misery on the welfare agency's millions of clients, according to the main workplace union.

The ongoing debt recovery debacle already has public servants at Centrelink stretched to breaking point, the CPSU is warning, and the agency's workers will soon have to cope with hundreds of thousands of student benefit applications and pensioners trying to make sense of changes to their payments.

But the department says customer demand is normal for this time of year.

The warning came on Monday as the Commonwealth Ombudsman announced it had launched an investigation into the debt collection project that aims to claw back up to $4 billion in overpaid welfare but has targeted tens of thousands of innocent clients for enforcement action over bogus debts.