New Zealand will give $50 million over the next three years to support Pacific fisheries.
Mr Key made the announcement at the Pacific Islands Forum in the Cook Islands today.
New Zealand already has $40m committed to the cause.
The extra $10m will go towards management of tuna resources, facilitating high quality investment and strengthening science in the sector.
"The stakes are high. The Pacific is the largest ocean on the planet not yet overfished."
Poaching is a major issue in the region and Mr Key said New Zealand would help with maritime surveillance as well as fisheries training and science.
"The region faces challenges to sustainably develop what is its single most important resource," Mr Key said.