The government will have to weigh up the costs of adding GST to online overseas purchases to make sure it's worth the effort.

John Key's announced work's underway with the OECD to try to add GST to online shopping.

Deloitte tax partner Allan Bullot says the government needs to figure out the most efficient way to collect the tax.

He says the government wouldn't want to adopt a tax that costs more to collect, than it brings in.

Allan Bullot says imposing the tax will require international co-operation.