Green Party energy spokesman Gareth Hughes has objected to the lack of public scrutiny for such a significant change and Auckland University law professor Bill Hodge said it was far from ideal to increase powers of arrest with minimal process.
Although there may be legitimate health and safety reasons for the law change, there was a general principle that the military should not be given law enforcement powers.
He said this should happen only if offshore mining sites and vessels were in waters police lacked the ability to patrol.
Mr Hughes said there was also concern about what constituted "interfered" with mining structures or vessels, a phrase which could be interpreted very broadly and open up relatively peaceful protesters to arrest.
Mr Bridges said it was not unusual for the Defence Force to take on enforcement duties in the deep seas, saying they were authorised to act as fisheries officers and customs officials in some circumstances.
The decision to make the change without public scrutiny would be debated at length in Parliament by all parties.
The types of protest action that would fall within the new offences included both physically causing damage or impeding the mining vessel to a "criminal" extent, such as going directly in front of it.
He said the changes were to clarify the law following cases such as that of Tauranga fisherman Elvis Teddy who was arrested after protesting against a Petrobras vessel exploring the Raukumara Basin. The District Court dismissed the charges because it took place outside the 12-mile limit, but that decision was overturned by the High Court.
New laws for protesters
Up to one year prison term or $50,000 fine for people who damage or interfere with mining structures or vessels (fine of $100,000 for organisations).
Up to $10,000 fine for breaching no-go zones of up to 500m around mining vessels
Police and Defence Force get new powers to board ships, arrest and detain protesters.
Apply only to protests outside the 12-mile territorial limit.
Change will be made as an amendment to the Crown Minerals (Permitting and Crown Land) Bill.