A mystery marine animal that has invaded Whangamata Harbour could threaten New Zealand's aquaculture industry.

The parasitic sponge-like animal, which kills all other sealife when it embeds itself on a surface, was found growing on the Whangamata wharf by harbourmaster Merv Martin late last year.

He asked for help from marine scientist Dr Brian Coffey, who had not seen a species like it in New Zealand waters before.

Samples have been sent to marine experts at Te Papa museum and the National Institute of Water and Atmospheric Research (Niwa), who were also baffled. It is now being examined by Dr Pat Kott, an Australian-based specialist in the field.

Since the animal's discovery, regional council Environment Waikato has commissioned Dr Coffey to study its "pest potential".

He found it had not only covered the wharf piles from the seabed to the low-tide level, but was equally abundant throughout the harbour, growing on 112 of the 130 mooring posts and on several anchored boats that were not often used.

Dr Coffey told the Herald that the animal was a "double invader" and had the potential to be a pest to the aquaculture industry like the fruit fly was to horticulture.

Destruction like that in the Whangamata Harbour could be disastrous if the animal took hold in a commercial mussel farm or something similar.

During its first invasion, the animal settles on a spot and begins a life "filter-feeding" on plankton.

But it will grow over and kill all existing sealife where it settles - from barnacles, limpets and seaweed to mussels, oysters and other shellfish.

Crabs and flatworms, which actually graze on the animal, are the only parts of the Whangamata Harbour ecology not dominated by its arrival.

Once its first hosts are killed, the animal peels away, exposing what Dr Coffey describes as a "morose sandwich" of the vile-smelling dead sealife to which it attached itself. It then begins the "second invasion", fixing itself to the clean surface and developing into thick, matted flat sheets.

The animal is a hermaphrodite and has the potential to reproduce asexually by "budding".

It has been temporarily dubbed Didemnum sp.? until it can be properly identified.

Early studies of Whitianga and Tauranga Harbour have found no evidence of the animal.

Dr Coffey said it could be months before the identification procedure was complete and, if necessary, a control strategy developed.

David Pearkes, Environment Waikato coastal manager, said it was keeping a watching brief.

The Coromandel Marine Farmers Association president, Gilbert James, said the animal might just be a "feral sponge" which had developed oddly within the unnatural harbour environment.

Mr James said 20 marine farming companies operated in the Coromandel. The area's aquaculture industry would double its growing capacity in the next two years.