An economist has found officials were given less than a day's notice of the fuel tax and Road User Charge reduction announced by the Government.

Documents show transport agency and ministry officials had less than 24 hours on a Sunday afternoon to cost and develop the policy.

The scheme was announced at the Prime Minister's post-Cabinet press conference the next day.

NZ Initiative Chief Economist Eric Crampton told Heather du Plessis Allan it was clear at that press conference and following briefings, they had no clue how to apply it to diesel.

He says the reason journalists couldn't get any detail was because officials were still trying to work it out days later.

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