A Ministry for Primary Industries trial has found grass carp kept in enclosures are effective in eliminating early infestations of aquatic weeds.
As part of the trial, six 2.5 metre by six metre cages were placed in beds of hornwort weed in Lake Karapiro.
Within nine weeks, the three to five fish contained in each cage had removed the weed.
MPI senior adviser Andrew Bell says once the hornwort was eradicated, the fish and the enclosures were removed safely.
"Grass carp in New Zealand cannot breed, so in terms of creating a sustaining population, there is unlikely to be a risk.
"However they would live 20 plus years, and that can be a long time of them feeding on aquatic weeds. We'd really like them to be in there, remove the pest weeds and then be out again and get on with having the native vegetation in place."
He said the cage design was effective in containing the fish. "We're pretty confident the risk of loss is very low," he said.
All but one of the carp was accounted for and Mr Bell said that fish may not have survived the transfer to the enclosure.
Mr Bell says caged grass carp can now be used to control hornwort if it's found in South Island lakes.