The government should give Holidays Act guidelines to companies and payroll providers, says the business which first alerted it to mass underpaying.
BP spokeswoman Shelly Braidy said it discovered the problem in 2012 and told officials the problem could be more widespread.
Since then investigations by the Labour Inspectorate have revealed about 24,000 employees have been underpaid across a range of companies.
One payroll software director, Matthew Gardner, said it was also possible the problem was much more widespread than that.
Ms Braidy said BP managed to catch it early and staff had now been paid their full entitled amount.
"It's a very complex issue, the Holidays Act, and we think that helpful guidelines provided to payroll providers and potentially employers about the Holidays Act would no doubt be welcomed," she said.
Several government agencies and companies, including the country's biggest dairy co-operative Fonterra, had been underpaying their staff.
A Labour Inspectorate spokesperson said people could have been underpaid by an average of up to $1800 per person.
Fonterra Managing Director Global Operations Robert Spurway said the fact so many large businesses had been affected showed the complexity of the Act.
"We will be working closely with our unions and consulting with other organisations impacted by the Holidays Act to reach a resolution, and to ensure that all Fonterra staff receive the payment they are entitled to," he said.
Payroll Practitioners Association chief executive David Jenkins said providers were feeling bullied and intimidated by officials who were failing to advise them on how to comply with the Holidays Act.