Waikato Regional Council has been forced to extend its deadline for applicants wanting to join the Healthy Rivers Wai Ora panel after forgetting to notify almost a third of the people on the ministry's list.
The council had agreed the registration of interest be sent to all 1145 Making Good Decisions Certificate Holders on the Ministry for the Environment website - but said the notification was "inadvertently" only sent to 806 people on the non local body elected list.
The error was first pointed out by regional councillors.
The healthy rivers plan, which is aimed improving the water quality in the Waipa and Waikato Rivers, received more than 1000 submissions and surpassed any other regional council proposal.
Waikato Regional Council director of science and strategy Tracey May said the initial request left out instructions and the registration of interest had now been sent to all holders, including the 339 local body elected holders who had been left out.
The deadline has been extended to May 19 to give everyone time to respond.
A MfE spokesperson confirmed WRC had initially only asked it to send the invite to all non-local body members who hold the Making Good Decisions certificate.
The council has budgeted $480,000 for the hearings.