Land in Mid Canterbury between Tinwald and Methven could be monitored for two or three years following the spilling of contaminated grass seed.
The spilt seed was potentially contaminated with a small quantity of viable seed of black grass - a pest plant not found in New Zealand.
Black grass, also known as slender meadow foxtail, is an invasive plant that affects winter crops in Europe, including winter wheat, grass seed, rapeseed, forage legumes and barley. It has developed resistance to many herbicides used for grass weed control.
The seed was found in a consignment of red fescue grass seed imported from Denmark. It was being taken to a containment centre in Methven when some was spilled.
John Wright, seeds group general manager of the importing company, PGG Wrightson, said the company was co-operating with the Ministry of Primary Industries investigation.
The ministry was also working with Federated Farmers, the Foundation of Arable Research and Environment Canterbury to address any biosecurity threats.
Mr Wright said the seed was quite immature but it could germinate in November, or lie dormant until the following November.
The ministry has said there was only a low chance that the black grass would establish from the spill.
The consignment was weighed in Denmark and weighed again when recleaned in Methven. A discrepancy of 47kg was discovered. Seeds were found on the deck of the truck which carried it and it was realised there had been a spill.
It was estimated there could be 6.5g of black grass seed in the spilt seed - about an eggcup full, Mr Wright said.
The contamination was discovered when the seed was held in the company's bond store. It was decided to reclean the seed and it was transported from Rolleston to Tinwald in a covered truck.
The mistake had been in moving it from Tinwald to Methven in an uncovered truck, he said.
The company could be prosecuted for the breach.
Much of the route of the spill is arable.
The commercial operators who imported seed contaminated by the pest plant should be fronting up to sort out the giant blunder, according to a property owner on the route.
Mark Lemon, whose property fronts about 800m of the route, is appalled the shipment was allowed to leave the wharf and then transported unsealed.
''I thought our ports were like Fort Knox. Why did it ever leave the port? If they didn't know about it until later, why wasn't it sealed in plastic before being transported?'' he asked.
The first Mr Lemon knew of the problem was when he heard a news bulletin on radio.
''The letter arrived the next day but the spill was a couple of months earlier.''
Property owners have been urged to keep the area under surveillance but Mr Lemon believes there are still many questions to be answered.
''Is it only spread by seed? What happens if it falls at the side of the road and is picked up in the cleat of a cow's hoof?
''They need to shape up. They can't put their head in the sand. If it means walking the land with thistle-grubbers, then they should be doing it.''
- by Maureen Bishop