Labour claims the Government has kept secret the real risk of a major oil spill at Anadarko's proposed Kaikoura drilling site.
Labour Leader David Cunliffe says documents obtained under the Official Information Act show Environment Minister Amy Adams had international research 13 months ago that shows there would be a 70 per cent probability of a `reportable incident' at the 1500m-deep Kaikoura well within a year of it opening.
Reportable incidents covers fire, vessel collision, injury, death, or pollution, including oil spills.
They were seven times more likely at the depth proposed off Kaikoura - and Raglan where a similar operation is planned - than the 300m depths drilled to off Taranaki, Mr Cunliffe said.
Mr Cunliffe alleged Ms Adams went to "great lengths'' to keep the information from the public, telling Parliament there was a "very low'' risk of a large scale oil spill occurring.
"This is a deliberate attempt by a relevant minister to deny the public information which is absolutely crucial to a mature debate on this issue.
"To know it is seven times more likely to have a serious incident on a deep sea exploration drill than an inshore drill is a very important fact to know.
"The Government needs to be honest about the risks of deep sea oil drilling, especially when the Kaikoura community and the wider public hold serious reservation's about Anadarko's plans.''
Mr Cunliffe said the information, which he released at New Brighton Pier in Christchurch this afternoon, was of "vital national importance''.
"A major oil spill would affect our international reputation, not just our environment, and harm our exports and tourist operators.
"We can't afford to risk our future for a short-term gain. If we blow a well head off Kaikoura, it will take weeks or months - probably months - to get a back-up rig there to help plug the gap.''
The documents shows that up to 90 per cent of the wells have a worst-case discharge rate of 100,000 barrels, about 16,000 tonnes a day, but some could discharge up to 350,000 barrels.
"And a couple of months' worth of major spill - unlikely though that may be - would be a significant disaster for wildlife, for the health of our oceans, for our fisheries and for our tourism brand at a cost of billions of dollars to New Zealand.''
Ms Adams said the figures Mr Cunliffe referred to were misleading.
"Clearly the risk does increase when you get into deeper waters, we've always said that. But to present that as a 70 per cent chance of an oil spill is just straight scaremongering.''
She said that in the Gulf of Mexico there were 2.5 incidents for every 1000 wells, many of them minor, and this figure was a more accurate reflection of the risk in Kaikoura.
"The best international evidence we have suggests it's at a very low risk of happening ... The risk exists, but we've never hidden that fact.''
Energy Minister Simon Bridges said that since the international report was published 13 months ago there had been significant legislative and technological advances in New Zealand.
"We've dramatically overhauled our law, our operations and technology has vastly improved - and we have got a High Hazards Unit. I'm satisfied we [have] world's best practice, we are safe and environmentally in a good place.''
Mr Cunliffe said Labour was "not opposed in principle to responsible and environmentally sensible'' offshore exploration.
But any consent to be granted under a government led by him would need world-class environment standards, top clean-up capability, full liability cover, a fair deal for taxpayers and a high level of consultation.
Mr Cunliffe said the deep sea drilling industry was a "responsible'' one, and hoped for a "mature conversation'' with Texas oil giant Anadarko, which is behind the plans.