The Green Party's demanding to know how much of the government's $200 million spend on consultants for its roads of national significance was wasted on a scrapped new expressway.

Government documents, released to Radio New Zealand, show about $216m has been spent in the past three years on investigation and design work for five of the seven roads - with $200m of that paid to consultants.

The work has been carried out on SH1 from Puhoi to Wellsford, Auckland's Western Ring Route, the Waikato Expressway, Wellington's Northern Corridor and Christchurch motorways.

Nearly $92m was spent on work done on the Wellington corridor - including a proposed four-lane expressway from Otaki to Levin, which the NZ Transport Agency (NZTA) last week announced would be scrapped.

Green Party transport spokesperson Julie Anne Genter says the government has wasted a fortune on motorways that don't make economic sense and won't be built.

"National decided to build these projects for political reasons. It ignored the benefit-cost ratios that show many of these projects are not worth the money they would cost and it didn't do due diligence in investigating other, more productive uses for the transport budget," she said.

"Now, having spent $216m on planning, it is working out what the rest of New Zealand has known all along: these projects don't make sense and they're not worth what they cost."

Labour's transport spokesman Phil Twyford says the spending on consultants is irresponsible, and shows a "mad dash" by the government to get the roads designed and built.

"In a country the size of New Zealand you cannot build all of those motorways at once. Even if they were all to go ahead - and there's no certainty they will - the builds would have to be staged over a period of time.

"Instead of proceeding in a staged way, the government has gone on a spending binge, throwing cash at design consultants for these projects all at once," he said.