Hunter Environment Lobby say the dead carp could strip the oxygen out of the water, which would kill other fish in the river as well as native aquatic plants.

President Jan Davis said research conducted so far had suggested the plan was not as harmless as it appeared.

She said the effect on the rest of the ecosystem had not been documented, and that must occur before the government tries to cull the pest species. 

“They’re a huge problem in the river and they have been for a long time, but trying to get rid of them with the herpes virus may not be the answer,” she said. 

“Environmentalists are concerned about the impact the virus is going to have on the ecosystem once the carp are dead in the river.”

The highly-contagious virus, which will be rolled out as part of a nationwide plan to manage the species, stays active for up to three days.

The CSIRO is conducting tests to determine if other fish species are susceptible to the virus. It will also will be responsible for monitoring the spread of the virus after it is released. Initial research has shown the virus will not harm other fish or humans. 

It has been suggested that scientists will have to come up with new strains of the virus to knock out carp who are not susceptible to the initial roll out.

Ms Davis said the 70 centimetre carp Ellie Burke caught earlier this month at Clarence Town had ignited debate about the fish, which destroys water quality.

“People don’t realise what’s lurking beneath the water in our rivers until someone catches a massive carp and its brought to their attention,” she said. 

“They wouldn’t know what the carp were doing to the river.”