A Texas-based company has applied to prospect 1.6 million square kilometres offshore of New Zealand - including off the West Coast - in the biggest petroleum application seen in this country.

"I've never seen anything of this scale before," Green Party energy and resources spokesman Gareth Hughes said this morning.

The application from ION Geophysical was lodged in late September and is being evaluated.

The company did not respond to a query through its Texas office asking for comment.

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According to its website, ION was founded in 1968 and has about 1000 staff.

Mr Hughes described it as a "massive application", covering an area the size of the Greenland icecap.

He said the Greens would "absolutely" be watching its progress.

If the prospecting permit was granted, he said, it would involve seismic surveys, which were known to have an environmental impact on the likes of Hector's dolphins off the West Coast.

"Ultimately if it went to drilling, which is some way off and seems highly unlikely with the oil price, there's a big risk," Mr Hughes said.

"We would urge the Government to decline the application."

Mr Hughes said that with world leaders meeting in Paris to address climate change, the West Coast should diversify its economy away from fossil fuels.

The ION website says it develops technology to improve seismic image quality. It also says it helps reduce the environmental impact of seismic operations throughout the world.

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"ION significantly improves image quality and reduces the risks associated with oil and gas exploration and reservoir development and production."

A spokesman for the Government's NZ Petroleum and Minerals agency said if the application was granted granted, the actual "lines" the survey ship would follow were long and widely spaced.

"Seismic surveys are often in a single sedimentary basin. This survey is across multiple basins. It is about a 'big picture' view of the geology around New Zealand, and the relationships between basins - not pinpointing somewhere to drill a well.

"There would also be significant research value in this data, in that it would provide specific information regarding tectonic plates and the geological settings of New Zealand."