The New Zealand SPCA is undergoing its biggest structural overhaul in 135 years.

All 45 independent SPCA centres are set to merge into one unified organisation, which it believes will result in more animals being saved.

Acting chief executive Andrea Midgen says the move will help cut costs and improve services.

"I can't express how delighted we are with this move that will put more money into being able to deal with more animal issues."

Midgen said the SPCA's services are more important than ever.

"We've seen an increase in the amount of animal welfare complaints year-on-year and the number of animals that are coming through our shelters is not going down."

The decision is a result of two years of discussions, culminating in a vote yesterday. Midgen said it wasn't about centralising control or closing any branches.

"The focus of every SPCA across the country won't change; we want to help animals in need and support our communities," she said.

"As one SPCA we can achieve more for our country's most vulnerable animals than we could as a fragmented organisation."