The Government's proposed changes to employment leave will cost workers "billions in wages", says Unite Union.
Gerard Hehir, Unite's assistant secretary, said most payroll systems had been wrongfully accruing annual leave - which meant workers earned less than they should've been.
"The difference is literally billions of dollars. Te Whata Ora alone owes over $2.1 billion to its employees," he said.
Minister for Workplace Relations and Safety Brooke van Velden revealed on Wednesday the Government wants to change to a pro-rata system for sick leave, and an accrual system for annual leave.
That means part-time employees may have to negotiate with their employer if they want the same 10-day sick leave as full-time workers.
The previous Government increased sick leave from five to 10 days in 2021 for every worker, but under the new proposals, part-timers would only get five.
Van Velden told AM on Thursday she wants to make the law simpler and easier for businesses, saying that it would benefit workers too.
"People would be better off getting their full entitlements because business owners wouldn't be falling foul of the law," she said.
She said many businesses had been misinterpreting the law.
"A lot of companies I talk to know that they are not getting it right but they're not quite sure how to get it right."
But Hehir said even if they do get it right, billions of dollars of wages will be transferred from workers to employers.
"It is very on brand for an ACT politician, but not what a government supposed to be looking after the interests of everyone should be doing.
"The 'complexity' often complained about is largely to ensure workers get paid the same while on holiday as they would if they were working."
Māori, Pasifika, youth, women, and other low-pay workers will lose the most, Hehir said. Meanwhile, those who increase their work hours within the year will "get screwed under an accrual system".
In addition, part-time workers who later take on more work hours won't get their full leave entitlements for the extra hours picked up, Hehir said.
"Other workers who will lose are those with higher hourly pay rates from overtime, penal rates, shift allowances and commissions."
When it comes to sick leave, the proposed changes will affect women the most, because "58 percent of women in paid work do less than 40 hours a week", said Hehir.
They also tend to use sick leave to look after children when ill.
Hehir also argued workers in retail, hospitality, education, and health will feel the brunt. They account for more than 60 percent of the part-time workforce.
"These are exactly the workers who are most likely to catch whatever illness is going around the community."
He said the 10 days sick leave for all employees is already straightforward.
"A pro-rata allocation will massively complicate it, especially for those [whose] hours vary week to week, because their average daily hours will need to be calculated as well as their pay rate."
He said those on variable hours, or working non-traditional 9am to 5pm hours, will lose out.
A draft bill to amend the Holidays Act (2003) will be released in September.
