Official advice that appeared to show the risk of an oil spill off Kaikoura was far greater than first thought is deeply misleading, Environment Minister Amy Adams says.

Labour leader David Cunliffe claimed that documents obtained under the Official Information Act undermined the National-led Government's assurance that deep sea oil exploration off New Zealand's coast was low-risk.

A study based on Gulf of Mexico oil wells and provided to the minister last year showed that the risks of an incident massively increased at a depth of 1500 metres, which is proposed in the Pegasus Basin off the South Island's East Coast. It said that there was a 10 per cent chance of an incident within the first year at a depth of 300m - the level of exploration in Taranaki. When the depth was increased to 1500m, the risk rose to 70 per cent.

When challenged on this finding in the House yesterday, Ms Adams said the incidents in the study referred not only to oil blowouts but also lesser problems such as property damage, equipment failure or worker injuries.


She said the risk of a well blowout was closer to 0.25 per cent. This was based on a rate of 2.5 blowouts per 1000 wells in the Gulf of Mexico.

Texas-based company Anadarko has permits for oil exploration in the Taranaki Basin, Canterbury Basin and Pegasus Basin.

The Government advice also said most wells had a worst-case discharge rate of 100,000 barrels, or 16,000 tonnes a day.

The Rena shipwreck in Tauranga in 2011 led to a leak of 2500 tonnes of oil. Maritime New Zealand has set a limit of 5500 tonnes as its response target.

The document said that rapid containment played a crucial role in limiting the scale of a spill, and noted that New Zealand was isolated from other drilling sites where capping and containment infrastructure was used.

Mr Cunliffe accused the minister of keeping the document secret but Ms Adams said it had been available online for two years.

Ms Adams' office did not initially release the drilling risk analysis, ruling that it was out of the scope of Mr Cunliffe's inquiry.

The minister was also challenged in the House yesterday about whether the Environmental Protection Authority (EPA) assessed the response plan which Anadarko submitted as part of its application for an exploratory well in Taranaki.


Ms Adams said that the EPA only had to assess the "completeness" of an application and not its effectiveness.