An email campaign against the proposed Ruataniwha dam in Hawkes Bay has infuriated local councillors, who say their inboxes have been swamped.
Regional councillors received at least 4000 emails from Greenpeace supporters.
Some were able to sideline the messages into a spam file but others were angry, saying their computers were deluged and their privacy breached.
Greenpeace agriculture campaigner Genevieve Toop said the organisation had encouraged its supporters to send the emails.
Councillor Peter Beaven said the swarm of emails was annoying but he was able to deal with it.
Others, including Alan Dick, were seething.
"It cost me half a day and it is going to cost me another half day to fix it up," Mr Dick said.
"It just shows that an organisation in this day and age can use electronic media to effectively invade people's homes and attempt to intimidate them."
Councillor Debbie Hewitt said she received thousands of emails, at a rate of every 10 seconds.
"It's an absolute invasion," she said.
"Your emails are not working, and you are unable to be doing any other work because you are trying to sort the jolly thing out. I have to manually delete each of these emails and it is just a jolly nuisance."
But Ms Toop said Greenpeace had done nothing wrong.
"This is not a spam attack, this is individual people raising their concerns about the Ruataniwha Dam because it is going to mean more industrial dairy farming and more risks to our water quality, which we simply do not need."
The Ruataniwha Dam would store flow from tributaries of the Tukituki River to supply water to a region that is notoriously dehydrated in summer.
But critics have said it would degrade the quality of water downstream.