Leaders of 15 Asia-Pacific nations have agreed to terms on the world's largest trade deal.

The Regional Comprehensive Economic Partnership (RCEP) was intended to include all 10 South-East Asian members of the ASEAN alliance, as well as Japan, China, South Korea, New Zealand, Australia and India.

However, Indian Prime Minister Narendra Modi has refused to join others in agreeing to terms, as he is battling domestic concerns the RCEP would undermine an already-faltering economy.

If India had joined the deal, the RCEP would have covered 32 per cent of the global economy, and half the world's population.

As it stands, it covers about 29 per cent of the global economy.

Australia already has individual trade agreements with ASEAN and the other countries in the deal, other than India.

The new pact formalises trade rules across the 15 countries and should allow better access to those markets beyond what the original deals allow.

“We have been through this sort of process with big trade agreements before in relation to the Trans-Pacific Partnership,” Trade Minister Simon Birmingham said before the announcement.

“Of course, we had hoped and expected the United States to be part of that. The door in the TPP remains firmly open to the US.

“We want India to be a partner here as well but we have to make sure that progress is realised amongst the 15 nations who are there without India.”