Moves are under way to stem a possible infestation of Japanese paddle crabs in the Ngunguru estuary.

A large number of the crabs (Charybdis japonica, and sometimes called Asian paddle crabs) were found there during a marine survey two weeks ago. It is the first time the species has been seen or reported there since nine adult crabs were found in the same area in December 2014.

Northland Regional Council (NRC) has notified local iwi, fishers, boat owners and other organisations that an invasion would put extra pressure on vulnerable shellfish stocks.

Japanese paddle crabs are aggressive predators and compete with native sea life for food; their main prey are bivalve shellfish.

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Their presence in the area where local pipi and cockle populations are already collapsing has spurred a two-pronged response from the NRC.

Aquatic biosecurity officer Irene Middleton said one approach will be to establish the size of the infestation in the estuary; the other to trial a trapping programme. The operation would be done with support from the local community as it would need to be a long term project, she said.

"We're trying to get an idea whether there's a population outside the estuary or if they're contained to the one site in the estuary. There are a lot of local fishers out there who would have been pulling them in - they're easily caught in flounder nets, for example - so we think we'd have heard before now if they're more widely spread," Ms Middleton said.

They are classified as an "unwanted organism" and cannot be gathered for food or taken away from where they have been caught, Ms Middleton said.

The species looks similar to the native swimming, dwarf and hairy red paddle crabs. It has a shell up to 12 centimetres wide, six spikes on either side of its eyes and flat back legs. It can be a variety of colours, from off-white, to light green, olive green and deep chestnut with purple markings.

It is more aggressive than the native paddle crabs that attract heavy numbers of fishers from Auckland's Asian community to Bream Bay beaches. Also slightly bigger, the Japanese variety would be more likely to latch on to someone's foot than just run across it, and to sink claws into and cling to a finger digging into sand in shallow water.

The species is widespread in the Hauraki Gulf and has been detected in small numbers in Whangarei Harbour and Bay of Islands.