The leader of a major trade union has unexpectedly thrown backing behind New Zealand First

While unions are traditionally deeply tied to the Labour Party -- First Union's general secretary has given a complimentary address at the party's convention today.

Dennis Maga praised the party's immigration views, citing concern about migrant exploitation, and Winston Peter's consistent support for greater ambulance funding.

Maga says other unions were in disbelief he had been invited to speak - but First Union has the most commonality with New Zealand First out of the coalition.

He says union members initially thought it was a joke - but are now pleased, and have asked Maga to advocate for the issues facing workers.