Seagull Islands hardly ranks as an obvious location for a spying operation designed to help police track terrorists. The outcrops in the elegant sweep of sand and water of Poole Harbour are populated by little more threatening than a colony of oystercatchers and large amounts of reed.

But four times in the past three years the islands and their surrounding waters have been under covert surveillance. Using boats, binoculars and CCTV cameras, a team from Poole Borough Council has been trying to catch cockle rustlers.

The local authority has been forced to defend itself against claims that it is Britain's nosiest council, because it has used the Regulation of Investigatory Powers Act (Ripa), a piece of legislation originally intended for protecting national security and investigating serious crime, to spy on fishermen.

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The Home Office says Ripa, which critics complain is now being used by nearly 800 public bodies in ways that risk infringing civil liberties, lays down rules for "using methods of surveillance and information gathering to help the prevention of crime, including terrorism". It was passed in 2000 to allow spying operations and monitoring of internet traffic.

Officials in Poole also confirmed they had authorised a two-week surveillance operation against a family wrongly accused of lying on an application form about their permanent home to gain a place at a popular primary school.

Jenny Paton, 39, her partner Tim Joyce, 37, and their three daughters had their movements chronicled by an undercover official who kept a precise record in a log book with entries such as "curtains open and all lights on in premises". The council exonerated the couple of any wrongdoing.

The episode brought a request under the Freedom of Information Act for the council to release details on how often it has used Ripa; that revealed 17 operations since 2005 to gather evidence on alleged infringements from damage to a road barrier to stealing from a rubbish tip.

The key project for the council's Environmental and Consumer Protection Service, which conducts the surveillance, has been enforcing a five-year-old ban on fishing stocks of clams, cockles and mussels in an area to the west of Poole town which includes Seagull Islands. Tests had found high levels of bacteria in the bivalves which could endanger health.

In surveillance operations averaging two weeks at a time, a team from the council along with Dorset Police and fishery conservation officials set to work. The council said the investigation was continuing. Tim Martin, head of legal and democratic services at the council, said: "We are committed to responding to the needs of local people who are concerned about crime, disorder and antisocial behaviour. Illegal shellfish dredging can cause harm to the conservation of stocks in the harbour and could also lead to a potentially serious public health risk if illegally fished stock is not fit for consumption."

Fishermen in Poole are unconvinced. One who has harvested shellfish in the harbour for 20 years said: "It's a hammer to crack a nut. If anyone is stupid enough to fish poisoned shellfish they will soon be discovered because of large numbers of people falling ill. I'm not sure they need to crawl around in the bushes like the SAS to prove it."

There is growing debate about the use of Ripa, originally framed to govern the interception of communications by nine government organisations including the security services and police, and by 792 public authorities including 474 councils.

A survey by the Press Association found that 46 local authorities had used the Act a total of 1,343 times in the past 12 months to investigate suspected offences ranging from benefit fraud and dog fouling to the sale of counterfeit goods and misuse of disabled parking badges.