Groups opposed to the Auckland Council's plan to drop 1080 in the Hunua Ranges to tackle a huge rat problem say it will be ineffective and the poison will end up in the local water supply.
The council said the rat population in the Hunua Ranges had ballooned ninefold over the summer, and it planned to drop 1080 on the area in July or August.
Despite activists' fears, some experts say the poison is still the best tool the country has to fix the problem.
Ban 1080 party leader Bill Wallace said the science for 1080 being effective was "shonky" and the toxin would not get rid of rats for long.
"They're such fast breeders, you'll find that the rat numbers will be four or five times higher next year, and they'll stay high for four or five years until the competitors and predator numbers build back up.
"The whole thing's nonsense. It doesn't work."
Mr Wallace said rats not killed by 1080 would be more wary of the poison in future and more likely to survive.
"If you poison again shortly afterwards, those animals that haven't died but have gotten incredibly sick, they've learnt not to touch that stuff."
The drop in the Hunua Ranges is near the city's water supply and Clyde Graf, who has made documentaries about 1080, said it would certainly land in the waterways.
"We've proven that at the regional council just this year, by investigating toxic flight charts," he said.
"At two kilograms of bait per hectare, there's enough poison to kill 80 deer ... it will kill, I think, around 16 people," said Mr Graf.
But Canterbury University professor of toxicology Ian Shaw said if the drop was done properly, the risk of the poison getting into tap water was very low.
"If, say, 20 or 30 pellets dropped into a water system, that would be a very very small amount of 1080.
"Still very toxic of course, but when it's diluted in the water system and the bacteria's had a go at it, the chances of any of that getting through to the tap in a house is very very low indeed."
Auckland Council biodiversity manager Rachel Kelleher said they had strategies to make sure that remained the case.
"Whilst it's unlikely to be a requirement to treat the area in two blocks, or isolate the water supply, we're taking that added precaution to ensure that there's absolutely no risk to the water supply."
The Parliamentary Commissioner for the Environment, Dr Jan Wright, said 1080 was the best tool the country had to protect its unique and vulnerable species.
"You can get a rat rebound, it depends on the type of forest, how much food there is around. This drop is occurring in winter when it's colder, breeding will be slower, less food around. Yes, there could be a rebound, but it's still a much better thing to do than doing nothing."
She said 1080 was a poison, and needed to be treated with respect, but if it was done right people could put their fears to bed.
"I think for many people there's a memory of the past, when it was used a lot more loosely."
Auckland Council is talking to nearby landowners about the drop, but Franklin Ward councillor Bill Cashmore said people living in Hunua were not worried.