The union said it believes the non-negotiable nature of contractors' "take it or leave it" agreements with NZ Post may be unlawful.
A union representative for staff at one of the country's biggest bakery companies has accused their employer of "slavery".
Next time you can't find the bread you want, it might not be because others are stocking up and panic buying.
"The company has tried bullying tactics... and now it has tried the bribe of a big one-off payment."
The company says it can no longer guarantee the safety and reliability of its services.