Work starts to eradicate one of the world's worst aquatic weeds from Pāpāmoa.

The Ministry for Primary Industries has started work to eradicate salvinia, one of the world's worst aquatic weeds, from the Wairākei Stream at Pāpāmoa. Photo/Supplied.

The Ministry for Primary Industries has started work to eradicate salvinia - one of the world's worst aquatic weeds - from the Wairākei Stream at Pāpāmoa.

Salvinia is an unwanted plant and notifiable organism under the Biosecurity Act, meaning it is illegal to grow or share plants.

The Ministry, Tauranga City Council and the Bay of Plenty Regional Council have worked together to develop treatment options.

The preferred eradication treatment was to use mechanical removal rather than the usual method of spraying with the herbicide diquat. This would mean using suction trucks and diggers to remove the weed from the surface of the water.

"The technique is a first in terms of the size of the infested area but we do believe it is a viable and worthwhile option to trial," says Dr Erik van Eyndhoven, principal adviser conservation at the ministry.

"Should the method prove successful, we will have gained a new tool for managing Salvinia."

Local iwi Ngā Potiki was also on site to assist with operations during the week to monitor for eels during the suctioning and to ensure appropriate cultural processes were respected.

The eradication work is part of a national eradication programme managed by the ministry under the National Interest Pest Responses programme.

The initial removal work would be followed by several monthly inspections to remove or treat any remaining plants. Even very small fragments of salvinia stems can regrow.

The inspections would continue until no further plants are found. The final stage is the monitoring period. Once there is a 2-3 year period clear of salvinia it can then be declared eradicated from the Wairākei Stream. This is expected to be around 2021/22.

The infestation is along about 1.4km of the stream that runs behind the housing area on Pāpāmoa Beach. The site is surrounded by residential properties, sports amenities and public access walkways and parks. The site is marked with tape and warning signs to remind people to keep away from the infested water.

Salvinia can completely blanket the surface, causing a drowning hazard.