While the state government has declared the ship is free of carcinogenic toxins, its testing for those toxins has been described by an international expert as "shockingly irregular" and "strange". Local protest groups are hoping to scuttle the scuttling for several reasons, including the suspected presence of toxins on the ship and a forecast 5.3 metres of permanent beach erosion at Avoca.

The government plans to sink Adelaide 1.87 kilometres off Avoca to create a diving and research site, becoming the sixth warship given to a state government for this purpose and the first in NSW.

When announced in 2008, the Adelaide reef project was welcomed as a recreational attraction and economic gift for the central coast. Sue Dengate, the secretary of the Central Coast Artificial Reef Project, which has been lobbying for 10 years, says: "It's going to draw divers from Australia and internationally, and will be a wonderful site for the study of marine life from cradle to grave. The majority of people on the central coast are right behind it."

But the mood changed in February when a report was released about the environmental effects of the scuttling. Protest groups are now calling on the federal minister responsible, Peter Garrett, to withhold his department's signature from the final "sea dumping certificate" necessary to authorise the scuttling.

Key to the protest is the possible presence on the 138-metre-long Adelaide of PCBs, or polychlorinated biphenyls, toxic insulator fluids banned in the US in 1979, a year after the ship was built in Seattle.

PCB exposure in humans has been linked to cancer, liver damage, skin diseases and lowered immune responses. As well as asbestos, CFCs, plastics, high-pressure cylinders and other hazardous materials, the PCBs on Adelaide were to be removed by the Adelaide company McMahon Services Australia at Garden Island, where the ship has been in recent months.

About 100 to 200 tonnes of material was removed from Adelaide during its clean-up. In its report on the preparation of the boat, the environmental services company WorleyParsons wrote in December: "Electrical cabinets and transformers are present throughout the vessel. These could contain PCBs or radioactive compounds. The [Royal Australian Navy] identified most of these items and installed warning labels to assist the ship preparation contractor in either removing hazardous material from the equipment, or removing the piece of equipment."

The non-specificity of this statement alarmed the No Ship Action Group, which asked the Minister for Lands, Tony Kelly, to guarantee that there were no PCBs left on the ship. He told Parliament on February 24: "I have been advised that all PCBs, lead and other toxic materials have been removed." Subsequently his department commissioned the company Airsafe to test samples from the ship for PCBs on March 1.

The testing found no PCB contamination, but the Herald has been told that Envirolab Services, which did the testing, was unable to sufficiently clean and test some of the samples and was not told where on the ship the samples came from.

The five samples were 10 centimetres by 10 centimetres and were not necessarily from the danger areas of the ship, such as communications, weapons and engine areas. The samples were supplied by McMahon, rather than coming from an independent source.

Colby Self, of the Seattle-based Basel Action Network, which monitors warship clean-ups, says: "I was shocked that there were only five samples taken. It's shockingly irregular compared to the testing done on similar ships in the United States. The samples were taken from insulation, which does not commonly contain PCBs, rather than electrical wiring, which does." He says only six PCB variants were tested for, whereas 209 can pose risks.

Two American warships of the same class, built at the same time as Adelaide, have been tested for PCBs in the US, Self says. "The ex-USS Estocin and the ex-USS Oliver Hazard Perry were both found to have PCBs in their electric cabling. The assumption for environmental remediation in the United States is that unless proven otherwise, you will expect to find PCBs, so you will sample widely. It doesn't seem that they have followed these protocols in Australia."

He says until further remediation and testing are done, there is enough evidence for a delay to the signing of the dumping certificate.

The director of public health at Northern Sydney Central Coast Area Health Service, Dr Peter Lewis, however, wrote in an email to protesters that, when Adelaide was built, the only permitted use of PCBs was in closed systems, and these had been identified and removed. "From a public health perspective it is very difficult to mount a case of health risks arising from environmental contamination," he wrote.

Dengate, who visited the ship on Wednesday, is satisfied that the ship is "clean as a whistle".

Quentin Riley, from the No Ship Action Group, who was on the same visit, was not convinced. "I couldn't believe that this boat was ready for scuttling. There were sheets of fibreglass matting on the walls that will be ripped out of the ship in the first strong swell. They assured us that there were no PCBs on the vessel but they are working to a tolerance level that is greater than zero. We think that zero is the only safe level for PCBs."

The action group has numerous other objections to the scuttling, including concerns about its effects on surf conditions, the stability of the beach and waterfront housing, the safety of the wreck itself as a dive site, and potential traffic congestion. It also objected to the placement of the ship off Avoca Beach, rather than Terrigal, which its members believed was the site originally announced.

Riley says the coastal engineers Lex Neilson, on whose original report WorleyParsons relied, and Professor Ian Goodwin, of Macquarie University, "confirmed a serious level of threat to the beachfront, meaning that 5.3 metres of beach could be washed away".

A spokesman for Kelly says there has been extensive community consultation, and the site was named as Terrigal rather than Avoca because boat access to the wreck would be from Terrigal. He says all testing for PCBs had shown the ship is clean.

McMahon Services, which has cleaned up warships for previous scuttlings including the former HMAS Hobart, is barred by its contract with the NSW government from commenting on its work on Adelaide.

Four inspections of the ship have been carried out by the federal environment department's independent expert technical adviser, which cleared the vessel of PCBs, a spokeswoman said.