Nine countries from around the region, as well as Australia and New Zealand, are in Tonga to sign the PACER Plus agreement, wrapping up nearly a decade of negotiations.
The deal has been polarising with critics saying it threatens the interests of island countries, and was unbalanced towards the interests of Australia and New Zealand.
But proponents tout it as a new kind of deal that links development to trade.
Todd McClay said this would give funding to boost exports from Pacific countries, and create rules to make it easier for the region's countries to trade.