A weed reckoned to be one of the world's worst has been discovered in the Whanganui district.

This comes after Horizons Regional Council biosecurity staff and Ministry of Primary Industries personnel inspected about 125ha of fodder crops over Easter weekend and found at least 252 plants.

Inspections also found signs of the velvetleaf weed in Ruapehu and Horowhenua.

Velvetleaf is classified as one of the world's worst cropping weeds. An annual broad-leafed weed, it grows 1m to 2.5m tall and is distinguishable by its large and heart-shaped leaves that are velvety to the touch.

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It was thought to be confined to the South Island but the weekend inspection showed it has spread. Meanwhile, MPI was investigating how the weed seeds could have entered New Zealand.

Bill Martyn, Horizons biosecurity manager, said the inspections included all known sites of the Kyros line of fodder beet which has been implicated in the velvetleaf "incursion".

"Unfortunately, the majority of these sites, except in Ohakune, are in the advanced stage of shedding seed. And despite staff removing all seed they could find, the rain on Friday and Saturday nights has washed many away and into cracks in the ground."

He said more worrying was the fact that while all known sites of Kyros fodder beet had been examined, there may be other lines that have been affected.

"Our staff is investigating other fodder crops, but as we cannot be everywhere at once, we urge farmers to check their crops for velvetleaf with its distinctive large heart-shaped leaves. We've found it to also look like a dead thistle plant at sites in our region," he said.

Horizons staff would be contacting contractors and seed reps to isolate and identify the risk areas of farms.

"Our staff will also be working alongside farmers with advice for site management into the future. We understand they will be concerned about land use. We'll do everything we can to help," Mr Martyn said.

The Ministry is currently investigating how the weed seeds could have entered New Zealand and working with a range of primary sector organisations and regional councils to establish the extent of its spread. It is an unwanted organism under the Biosecurity Act, meaning its entry to New Zealand is prohibited.

He said any farmers with questions or wanting help checking their crop should call 0508 800 800.

"However, if you think you may have found the plant, please make immediate contact with MPI via tollfree number 0800 80 99 66," he added.

-Velvetleaf is Asian in origin but has the potential to be hugely damaging to crops.

Since being introduced to North America in the 18th century, it had become an invasive species in agricultural regions and costs hundreds of millions of dollars a year in control and damage. It is usually controlled by herbicides.