It's been referred to as the world's number one corn-killing pest and was first detected in Tauranga in March last year.
Now it's already travelled south, spotted in Mohua (Golden Bay), leaving arable farmers and vegetable growers concerned.
"It's gotten as far as North Canterbury and the West Coast of the South Island, so it's been really prevalent in the North Island," Colin Hurst from Federated Farmers told 1News.
The fall armyworm can feed on more than 350 plants and has its name because the insects gather in huge masses, decimating crops.
"We've managed to monitor it - this past season, there were about 140 sites around the country.
"It's called an armyworm because the numbers build up in the autumn, as in the fall, and they march across the landscape like an army marching," Hurst said.
It's native to the Americas and since 2016 has spread to Africa, Asia, and Australia.
With crops under attack, the fall armyworm is costing billions of dollars in lost production across Asia.
Arable research experts have said we won't be able to get rid of it.
"There is a way to kill them, but now that they are in NZ, they are here to stay," Hurst said.
"We have got chemicals to control it. Luckily MPI fast-tracked some chemicals, so at least we've got some chemical trials.
"It's not ideal, we're just hoping we can get on top of it.
"It's actually terrible, but the other concerning thing is that it could affect our dairy farmers because New Zealand grows about 1.2 million tonnes of maize solid."
Hurst said about half of that is in the Waikato and Northland regions, which could have a stifling effect on the feed for dairy farmers.
In a statement, the Ministry for Primary Industries (MPI) said they ran a biosecurity response from April last year in an effort to contain and eradicate the pest.
But by April this year, it became clear fall armyworm was widespread.
"This included surveillance and research to better understand this plant pest, its spread, and potential impacts in New Zealand.
"Eradication was unlikely because it had been windblown from Australia, and this is likely to repeatedly occur," MPI said.
Industry groups have now adopted a long-term management program.
"We haven't seen the full impact yet, the full economic damage of fall armyworm yet in New Zealand," Ivan Lawrie from the Foundation for Arable Research told 1News.
"What there is, is a lot of confusion and misinformation out there.
"There are other caterpillars out there that attack maize and sweet corn crops," he said.
The cosmopolitan armyworm, tropical armyworm, and the corn earworm have all been established in Aotearoa for many years.
"I think the advice is to get a very clear identification."
Lawrie said they had 250 traps deployed all over New Zealand, in and around maize and sweet corn crops.
"We started to detect the flights across the country, but more importantly, the way to monitor this is by going into the crops and seeing the actual damage," he said.
It's hoped Aotearoa's cold climate can help mitigate the spread.
"It would appear that there are some warm conditions at the moment, and that would promote survival. However, if we do get some frost, then we know that fall armyworm does not survive prolonged frosty conditions," Lawrie said.
They have modelling that explains where it's most likely for fall armyworm to survive, indicating that the regions north of Waikato, Northland, and Auckland are where it is more likely to survive the winter, he added.
"There are pockets within New Zealand that are also frost-free, such as the West Coast of the South Island.
"In certain years, we'll have a larger number of fall armyworm attacking the crops, and in colder years, that number might contract."
Lawrie said serious work is underway to understand where the pest is, how it can be controlled, and what the potential economic damage and controls are that we need to understand.
He said that, at the early stages of the crop next spring, people should be walking their paddocks and checking for caterpillars.
"Get them early before the caterpillars grow too big or the crop grows big, and there are very clear signs of fall armyworm damage."
He said we always need to be concerned when new species come and invade New Zealand.
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