The Ministry for Primary Industries is investigating reports that commercial vessels are fishing in a marine sanctuary in Banks Peninsula.
Conservation group Earthrace said it saw up to seven boats fishing in the Banks Peninsula Marine Mammal Sanctuary during covert surveillance it carried out earlier this year.
The sanctuary is home to the endangered Hector's dolphin.
The surveillance involved observations from a plane, motor boats, kayaks and hilltops around the peninsula over a 10-day period.
Earthrace founder Pete Bethune said fishing boats steered clear of the sanctuary during the day, but at night moved in and began gill netting and trawling - techniques known to trap and kill dolphins.
Mr Bethune said the group is hoping its evidence will lead to a prosecution and urged the Government to look at protecting these sanctuaries better.
He is calling on the ministry and the Department of Conservation to extend the sanctuary from 12 to 20 nautical miles and better protect the dolphins. However, DoC said on Wednesday it satisfied with the level of protection and had no plans to extend the boundaries.
The sanctuary covers more than 400,000 hectares and extends from the mouth of the Rakaia River to the mouth of the Waipara River, according to the department's website.
As well as restrictions on commercial set netting and trawling, amateur set netting is banned and seasonal set netting for flounder is permitted only in designated areas from April to September, it said.
The Ministry for Primary Industries said an investigation is under way and the issue is being taken very seriously.
Christchurch's Pegasus Fishing is one company that has been questioned as part of the ministry's investigation.
Managing director Tony Threadwell, who has fished off Canterbury for 35 years, said on Wednesday that neither his boats nor those of any other company are fishing illegally.
Mr Threadwell said some boats either rest or travel through off-limit areas, but only use set nets and trawlers beyond the boundary.