The Engineering, Printing and Manufacturing Union says it will monitor how Air New Zealand's new baggage allowance affects its members.
As of Monday, domestic travellers will be allowed to check in bags weighing up to 30 kilograms instead of 20kg.
A $10 charge will be imposed on any additional luggage.
The union was initially concerned about how baggage handlers would cope with the extra weight, but it says the airline has taken steps to deal with safety aspects of the changes.
It says new technology has been introduced to save workers from having to load aircraft by hand.