Black-grass has not yet been discovered in Mid-Canterbury following three rounds of surveillance after a truck spill, the Ministry for Primary Industries (MPI) says.
More than 2000 seeds of the invasive weed was spilt from the back of a truck between Ashburton and Methven in July.
The seeds of black-grass, or meadow foxtail, an invader of winter crops in Britain and Europe, were found in a consignment of red fescue grass seed imported from Europe.
Ministry contractors are carrying out surveillance between Methven and Ashburton in response to the red fescue seed believed to be contaminated with black-grass spilt along the route.
A further six rounds of surveillance were scheduled in the next five months, and contractors had applied herbicide to areas thought to be high risk for black-grass to establish, ministry response manager David Yard said.
A second round of treatment was set for March next year, he said.
"It's all going very well, but we're not out of the woods yet," he said.
"We've got another two and a half years of surveillance and treatment before we can start calling the response a success or not."
Black-grass has developed resistance to many herbicides in Europe.
The Government is working with the industry to ensure the pest does not establish in New Zealand.
MPI is leading the response, supported by Federated Farmers, the Foundation for Arable Research, Environment Canterbury, the Ashburton District Council, the New Zealand Grain & Seed Trade Association and PGG Wrightson.