The NSW opposition says secret emails show the state government considered hiding the administration fee motorists must pay when they claim a refund for their compulsory third-party green slip.

Following an overhaul of the CTP car insurance scheme last year, most motorists are eligible to receive a partial refund on their green slip. But they also have to pay an administration fee.

The green slip refund is one of the government's much-touted cost of living measures.

More than 2 million NSW vehicle owners have claimed $128 million worth of refunds. Credit:Anthony Johnson

But emails obtained by the opposition under freedom of information laws show there was discussion within Service NSW and other government agencies about whether the $7.87 fee should be disclosed to motorists.