Some workers would be getting several thousand dollars but the average payout was expected to be $588.
Postal Workers Union of Aotearoa spokesman Graeme Clarke said most of those owed money would be unaware of the windfall and not realise they had to apply to NZ Post.
Many former staff had moved house so there was concern letters and the money would never reach them. NZ Post will place advertisements asking former staff to get in touch.
"Unfortunately, there's not a lot that can be done for that kind of issue which is why the advertising is going to take place. People who have left NZ Post will need to come forward to seek payment."
Former employees would need to provide documentation, including proof of identity and address, Inland Revenue Department number and bank account details.
Current employees will see their share of the $11.7 million backpay for existing staff being deposited into their bank accounts within the next eight weeks.
Some workers who had been with the company over the 10-year period were expected to receive lump sums worth several thousand dollars.
Said Clarke: "It's quite significant for people who have fought for it. The claim was first raised in 2008 and we attempted to negotiate with the company. It's taken a long time to go through the court process."
Clarke said it was worth any past employee checking if any money was owing.
NZ Post 'could have saved a lot of money'
Former postie Linda Street pedalled the streets of Christchurch for more than five years on ever-increasing sized rounds, clocking up unrostered overtime.
She didn't think she wouldn't be paid for her work until, one Labour Weekend, she questioned why her paypacket didn't show similar returns as her partner's.
"I realised he was getting paid extra because he was working overtime and I thought, 'I've been working a lot of overtime and they're not paying me any more'."
For a year, NZ Post "led me to believe they would pay it. Then they changed tack and went to court."
The Postal Workers Union lost the two first court cases, but finally won in the Court of Appeal.
Before Christmas, Street became the first former employee to receive backpay, of $1400.
She criticised NZ Post for fighting on, as it would have cost much less if workers had been recompensed from the outset.
"At the time, it seemed a minuscule amount of money that I was rabbiting on about. But then when I was looking at it I thought, 'Why shouldn't I have that $10?'"