The risk from the pest weed Velvetleaf to this country's cropping farms can't be underestimated, a biosecurity expert says.
A frantic search is underway in parts of the country for any signs of the plants before they seed and spread.
The pest plant has been identified in imported fodder beet seed.
Velvetleaf has been found in Waikato, North Canterbury, Central Otago and Southland, but over Easter 250 plants were discovered in 25 hectares of fodder beet in the Whanganui, Ruapehu and Horowhenua districts.
Farmers who planted Kyros and Bangor varieties of imported fodder beet seed are at real risk that the weed is growing in their crop.
Bill Martyn from Horizons Regional Council said in the latest find most of the plants were in an advanced stage of shedding seed.
"It's not very good and we have had a lot of rain over the weekend which helped compound the problem and dropped the seed and washes it away and makes it hard to pick up off the ground. So certainly not good news."
Southland Regional Council biosecurity manager Richard Bowman said it was difficult to know what would happen if velvetleaf was able to take hold.
"The one thing that does scare us we believe these seeds have a life of up to 60-years in the soil, so it means it is going to be extremely difficult to get rid of them once they are there. It could be extremely difficult to contain and it would mean the farmers concerned would have to work under a fairly stringent farm management plan to prevent it from being moved around and obviously we don't want to get into that if we can help it, so if we can prevent this particular lot from seeding then hopefully we won't have to worry about it in the future, but if it does in fact seed then we have an ongoing issue to be dealt with."
Mr Martyn said velvetleaf had the potential to badly effect crops if it took off.
"We are hearing reports that it can restrict maize crops by 30-40-percent, so it is certainly not something you want to establish if you can help it."
The Ministry for Primary Industries is investigating how the weed seed could have entered New Zealand.