He said it was difficult at this stage to say exactly how many workers would ultimately be paid back money.
"It's too early to say who exactly may be impacted, but some scenarios that typically result in underpayments are those employees who earn regular overtime or have a variable pattern of work. We'll be able to confirm with individuals who register, and start the back payments from July 2020," Boersen said.
NZ Post is working with the union to contact former employees to make sure they receive what's due to them.
E tū negotiation specialist Joe Gallagher said it had faced challenges reaching some former employees.
"NZ Post has written to their last known address, but as it dates back to 2010, many people's details may be out of date," Gallagher said.
"So we're doing all we can to contact them, and urge Kiwis who know any former NZ Post employees to tell them about registering their details at the portal."
NZ Post reported a loss of $121 million in the 12 months ended June 30 this year, compared to a profit of $13m a year earlier when the bottom line was buoyed from its share of Kiwibank profits.
The latest period did, however, include the $38m being set aside to cover holiday pay as well as $51m impairment charge on mail service assets and a $15m settlement with ACC and the New Zealand Superannuation Fund.