Over 50 tonnes of material has been dredged from the Tasman Bay seabed off Nelson and disposed of on land after invasive brown mussels were found.
The South African mussels, known as perna perna, ended up in area of seabed beneath where an oil rig was cleaned latelast year.
Biosecurity New Zealand said two separate sites were dredged by a scallop dredge and the material, mainly dead shell and sediment, was disposed of at a local landfill.
David Yard from Biosecurity NZ said the risk of the small number of brown mussels present establishing a population was always low, but that risk had to be mitigated.
The brown mussel has recently invaded North America around the Gulf of Mexico and is rapidly becoming a nuisance for water-cooling systems for power stations.
An outbreak in Tasman Bay would pose a threat to New Zealand's $200 million mussel industry.
"We can now safely say that we are comfortable all reasonable efforts have been made to eradicate the brown mussel...from the affected area in Tasman Bay," Mr Yard said.
Any risk of it re-establishing were "negligible", he said.
The owner of the oil rig, Texas-based Diamond Offshore Drilling, is covering the cost of the dredging clean up, which stands at about $250,000.
Biosecurity NZ said it was planning to discuss the situation with key stakeholders in the region over the next couple of weeks.
