Green Party co-leader Russel Norman described the breach as "cowboy behaviour by the oil industry" and blamed it on "the Government's cavalier attitude towards environmental protection".
"National has created a culture in which environmental protection is not seen as very important and in which New Zealand rolls out the red carpet to the oil industry," he said.
"In this context, it's hardly surprising that a major oil company has broken our environmental protection law by drilling wells that the EPA says are illegal.
"We now need the EPA to pursue prosecutions so that the industry learns its lessons. As the EPA says itself: 'There have been breaches of the Act. If there is no action taken in response to the non-compliance, it is likely that the non-compliance would continue'."
He called for "much greater protections for our marine environment".
The EEZ Act came into force in June 2013, and the EPA said the extension wells were started after this date and without consent.
"It's inevitable with a brand new law, and we've never had regulation in the Exclusive Economic Zone around these activities, that there's a bit of argy-bargy about what is or what isn't outside the law," Environment Minister Nick Smith told 3News.
In a statement to 3News, STOS said it believed it started the work before the new law came into force, and it was therefore exempt.
"It is not uncommon with new legislation that there are different interpretations initially," it said, adding it believed the drilling had "no adverse impact on the environment".
