A vast swarm of bugs that covered much of the northern half of the North Island last night and this morning has been caught on the Metservice weather radar.

MetService didn't know what it was and entomologists were puzzled.

But there was a strong suspicion that it was an unwanted Australian heading home.

One was convinced it was the Tasmanian grass grub and another suspected huge swarm of aphids.

"I have no idea," said Bugman Ruud Kleinpaste.

"I would suggest go up in the air in aeroplane stick out a butterfly net and see what you catch."

He also speculated it could be a dust storm.

Fellow entomologist Stephen Pawson of Scion, a Crown research institute, had a more pragmatic answer; whatever if was, it was large, and it is going to be coming in on the surf at Piha and Muriwai for the next week or so.

Metservice's Peter Kreft said the unknown insects began swarming over the Waikato region about 9pm yesterday.

"They were pushed by the south easterly wind north toward Auckland," he said.

By 7am Friday the massive swarm was out over the Tasman Sea, west of Auckland.

Kreft said the weather service was not sure what it was.

"We strongly suspect the echo is swarms of insects," Metservice said.

"To show up in radar imagery like this, they must be about as large as, and as numerous as, precipitation particles....

"We see this from time to time, but this one is a very good example of it."

Pawson said at a guess the timings favoured Tasmanian grass grub (aphodius tasmaniae), an introduced agricultural pest.

"It is the right time of year for them to emerge. They can come out in massive numbers," he said.

"Its right time of year, right time of day and to a degree - and I am not an expert on Tasmanian grass grub - but we've had drier conditions."

He said the weather conditions had to be just right and they would have been influenced by the light southerly front.

A similar emergence had occurred in Canterbury some years ago and they all washed up in big waves on the beaches near Christchurch.

And another emergence occurred between Christchurch and Ashburton on the night of January 23.

Pawson said there had been a lot of cases of this overseas. The best known was the mountain pine beetle in the Pacific north west of America. That swam covered an area larger than that of New Zealand.

Hort Research says the beetles are 10-12 mm long and they make tunnels in the soil in which they stay during the day.

Its larvae feed on the foliage of grasses, clovers, and lucerne in pastures, home gardens, and recreational grassland and turf. They bite off pieces of leaf, which they drag to their burrows to eat later.

"Tasmanian grass grubs live in light soils in the coastal areas of Canterbury (particularly), Marlborough, Auckland, and Northland. Infestations tend not to cover wide areas, but are patchy and severe over relatively small areas of up to one hectare," Hort Research said.

"The beetles fly at dusk on still, warm nights between mid January and mid March.

"Dense swarms aggregating around street or shop lamps in urban areas are common."