The government has called off its programme to destroy the pest plant velvetleaf and said it was unlikely it could be eradicated.
The pest entered the country in contaminated fodder beet seed and there has been 244 detections of it in 11 regions so far.
Ministry for Primary Industries (MPI) incident controller David Yard said the current programme had been stopped and there was now discussion over the next phase of the operation.
"It's difficult to say what the cost of whole response will be because clearly we don't know what the long term management will look like and the interim arrangements until we've had meetings with stakeholders.
"The precise level of contribution I can not comment on at this moment because we just don't know."
Mr Yard said MPI was trying to work out who pays for the next phase and would have a clearer idea of the cost of the outbreak after a meeting of stakeholders in two weeks.