First Union is taking legal action on behalf of courier drivers, and will argue they are essentially employees rather than independent contractors.
The union has begun proceedings in the Employment Court to determine the employment status of courier drivers.
Most drivers are on independent contracts, which means they have to pay for their own vehicles, scanners and uniforms. They are also not entitled to sick leave or annual leave.
Many have told Checkpoint in recent weeks that they make less than minimum wage after tax and expenses.
First Union secretary for transport logistics and manufacturing Jared Abbott said many drivers contacted the union in recent weeks, following Checkpoint's stories.
"We have begun proceedings to file to the Employment Court to determine the employment status of a bunch of courier drivers and we believe with the new precedents being set that we'll be successful in being able to get that determination."
Employment Court's former chief judge Graeme Colgan told Checkpoint last week that it would be "worthwhile" for courier drivers to take a case to court, and said a union might be able to help with legal action.
Courier drivers said while they were on independent contracts, they were not allowed to work for any other courier company.
New Zealand Post confirmed this was its policy.
First Union will argue the drivers are employees, not contractors.
Mr Abbott said courier drivers made up a small portion of drivers in the same situation, and Mainfreight truck drivers were also on independent contracts.
"We're very confident we can win it, but the reality is even if we can't win the case we don't believe what is happening should continue, but what we're going to do now is a little bit different, where we're actually going to try and overturn that case law that was set in TNT Cunningham and show that these people may actually well be employees."
The Cunningham vs TNT 1992 case is one in which a courier driver took TNT Express Worldwide to court because he believed he was unjustifiably dismissed, but TNT argued he was not an employee.
The Court of Appeal overturned an Employment Court ruling, and sided with TNT.
Mr Colgan said the legal process would be slow and fiercely defended by the courier companies, and would likely go all the way to the Supreme Court.
Employers and Manufacturers Association chief executive Kim Campbell said courier drivers willingly entered into an arrangement which stated they were independent contractors.
"These people are not forced into this. Nobody puts them there at gunpoint. They made a choice to run their own business. If they don't like it they should sell it, get out and do something else."