The Nurses Organisation says Health NZ Te Whatu Ora is asking staff to pay it back money given in error, despite it owing current and former workers $2 billion in remediation payments.
It is one of the cost-saving measures Health NZ is using to make $105m in cut backs by July, as directed by the government, NZNO said.
A Health Canterbury job advert - posted two weeks ago - said it was looking for a "Legislation and Compliance Overpayment Officer" to join the region's corporate payroll team.
It was a permanent, full-time role that would "calculate overpayments in line with Health NZ policy" and "contact managers regarding sensitive overpayment situations and for recovery of funds".
The position was also responsible for "investigation and resolution of payments".
"It is common to have roles within payroll teams that are responsible for managing overpayments," Health NZ chief people officer Andrew Slater said.
"An overpayment may occur if an employee's hours of work change and the form to reflect this change is not processed by the employee, manager or payroll team within the relevant payroll period," he said.
Health NZ said the role was not part of cost-saving measures.
Staff had already been approached to pay back funds across the motu, NZNO delegate Erica Donovan said.
The payments had not been rectified in the past four or five years when it happened, but had suddenly come up this year, she said.
Donovan felt the move was disappointing and hypocritical when Health NZ owed hundreds of thousands of employees $2.1b in Holidays Act remediation payments for breaches around shift and leave pay, dating back more than a decade.
"We've been locked in a dispute with the hospital system for the last several years over payments being made incorrectly under the Holidays Act," she said.
"[Health NZ] has been allowed to get away with that for several years and we've been constantly told 'money will be coming' and it's not. Now they're chasing my fellow coworkers for money back."
To date, Health NZ had paid back $246.5m to clear 13 years of mistakes for 34,000 people who currently work in the Auckland districts.
The other 17 districts and former staff were due to be paid out in 2023, but Health NZ had pushed out that deadline to the end of this year.
A joint project to get it paid back, as well as bring in a unified national payroll system, reported last October that it was running short on resources to do the work.
Health NZ employed 90,000 people and was working with 24 different payroll systems, Slater said.
There were also 26 National Collective Agreements, 33 single collective agreements, plus about 10,000 people employed on individual agreements it had to manage, he said.
"Many of the payroll processes in Waitaha Canterbury, are manual, and this includes the calculation of over payments."
Any overpayment "would be recovered in line with the impacted employee's employment agreement", Slater said.
Leaving shifts understaffed, not replacing sick staff, changing brands or medical products or stocking less of them were also measures being implemented to reach the savings target, Donovan said.
"It's really unsettling when you're trying to provide the best patient care and not having the supplies or the staff to do so," she said.