A planned 1080 drop is causing controversy, with locals worried about the water supply and questioning why it needs to happen at all.
The poison pellets will be dropped over rural parts of the Waikanae and Reikorangi area north of Wellington, starting tomorrow.
The operation is being run by TB Free, which aims to reduce bovine tuberculosis by keeping the possum population down.
But Reikorangi deer farmer Peter Adlam said he was worried about the impacts of the poison, and wasn't able to get an answer about his fears from TB Free.
He said the 1080 pellets were being dropped only 100 metres from his boundary and he was worried his deer would be poisoned.
"[The deer] are used to eating pellets, they're fed pellets on the farm, and they're very valuable deer.
"That one I just got cost $5000. The last thing I want is for them to get 1080."
Adlam said he wasn't even sure why the 1080 operation was needed.
"I've asked TB Free themselves and I couldn't get an answer.
"Is that a good use of funds that farmers are paying for? There's no TB in the area, so why are they dropping it?"
Local Helen Naylor keeps sheep on 40 acres of land.
She said if poison was needed she'd prefer manually installed cyanide stations, instead of the aerial 1080.
But she was unsure why the poison was needed at all.
"As far as I'm aware, there is no TB in the area. And the regional council has been using brodifacoum over the last 18 months, and the number of possums is drastically reduced.
"So I'm not entirely sure on what this new 1080 drop is supposed to target.
"Both our water supply for the house, and water for stock, comes from a stream up above our property.
"The idea of having 1080 aerial dropped within that vicinity, I find that upsetting and concerning."
Naylor said she was worried the aerial operation would overrun the drop zone, and accidentally get some in her paddocks.
Ospri (Operational Solutions for Primary Industries) runs the TB Free programme.
An Ospri spokesperson said they'd notified landowners within the region, as well as making personal visits.
Two information sessions were held in the local area in early August.
He said monitoring of possum numbers showed more control work was needed in the Waikanae area.
"TB-infected pigs were recently found in the area in 2013, with previous TB-infected wildlife found dating back to the early 1990s."
Ospri said that to eradicate bovine TB possum numbers had to be kept extremely low, around one to two animals every 10 hectares.
The spokesperson said they'd already been in contact with both Adlam and Naylor.
He said they were working with Adlam to confirm the 1080 drop boundaries in order to address his concerns about the impact on livestock, particularly if any deer happened to escape his property.
Ospri said they'd already identified possibly sensitive areas, including water sources.
"It was identified early in the planning process that Kapiti water treatment station will need to be excluded from the operational area.
"It is important to note that 1080 is highly soluble in water. It naturally breaks down into harmless substances through the process of biodegradation and dilution.
"Extensive research has shown that it does not accumulate or leave permanent residues in soil, plants, water or animals."
A Kapiti Coast District Council spokesperson said the water supply was safe.
"The Ministry of Health hasn't advised us to take any extra precaution and we're comfortable that we have robust systems and processes in place to make sure the water supply we're delivering to our community is safe, clean, and drinkable."