The safety catch has been put on any duck hunting at a Far North wetland to prevent the accidental spread of an invasive aquatic weed.

The Department of Conservation (DoC) has closed public access to Te Paki's Te Werahi Lagoon during duck hunting season to prevent the spread of gypsywort and its high risk threat to other waterbodies and wetlands in the Far North.

Te Werahi Lagoon is the only place the weed is thought to be in Northland, and experts believe it was accidentally introduced from the Waikato by duck shooters.

Mike Finlayson, who represents the Northland Regional Council's (NRC) most northerly Te Hiku constituency, said while DoC manages the lagoon and access to it is across Te Paki Farm Park, the park's owners, Ngati Kuri, support the closure.

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Because it poses such a serious risk to wetlands, gypsywort has been flagged as an "eradication plant" under the NRC's Proposed Regional Pest Management Plan.

The weed - which flowers late summer and produces a lot of seed - is abundant in the Waikato and Rotorua Lakes.

Gypsywort is a mint-like plant originally from Europe and Asia, up to a metre tall, with square stems, toothed paired leaves and tiny white or pale mauve flowers produced in groups above each pair of leaves in the upper stem.

It has underground runners but despite its mint-like appearance, does not have a minty smell when crushed.

NRC biodiversity manager, Lisa Forester, said it forms dense patches and smothers native water edge plants such as raupo, reeds, sedges and rushes.

The seed can float and survive for more than eight months.

It is spread by water, footwear or contaminated machinery and can be dispersed by ducks after it passes through their digestive system.

Mr Finlayson said the danger of gypsywort being spread through wetlands highlights the need to follow the "Check, Clean, Dry" message.

"This means making sure all equipment used between waterways is thoroughly cleaned and dried for at least 48 hours before using it in the next water body or wetland."