Greenpeace says New Zealand's largest mining company Shell-Todd displayed wilful ignorance in breaching new maritime laws.
The Environmental Protection Authority has issued the company a slap on the wrist for not acquiring resource consent for a new side-well last year.
The Exclusive Economic Zone Act allows any existing wells to continue without a marine consent from the Environmental Protection Authority.
But, since the act came into effect last June, a permit had been required for any significant amendments or creation of new wells.
Shell-Todd Oil Services, or STOS, did not have marine consent when they created two new side-wells - horizontal extensions of existing wells used to tap new gas fields.
EPA manager of compliance Andrea Eng said Shell-Todd Oil Services should have had a permit.
"There is a provision in the act which allows operators to continue their existing activities but these particular side-track wells are considered to be new activities.
"So therefore they would have needed some form of approval from the EPA, whether it's requesting a ruling or a marine consent."
In a written statement, Shell-Todd said the company believed it was in the clear.
"STOS is a strong supporter of the aim and intent of the EEZ legislation however it is not uncommon with new legislation that there are different interpretations initially.
"In our view the activities referred to in the Māui A EEZ Post Inspection Report have had no adverse impact on the environment and we will continue to work constructively with the EPA to come to a common understanding of the new legislation."
Greenpeace's senior policy advisor Nathan Argent said Shell Todd displayed a cowboy attitude towards the law.
He said this should ring alarm bells about how offshore mining was conducted in New Zealand.
"Big water drilling and this kind of attitude towards regulations shows that rather than having the world's best practices as the Government claims, it's more like the wild west.
"We simply don't need to be taking the risk to do this kind of drilling.
"There are much cleaning, smarter alternatives New Zealand could embrace."
But Andrea Eng said it was not that simple.
"With any new legislation there's always going to be a period of time where there will be differences in intepretation between what the regulator and the operator thinks the legislation says. So, in this particular case yes there were differences in interpretation."
She said Shell Todd had been issued a warning letter for now - but further breaches of the law could result in a fine of up to $10 million.