Aramoho residents say they felt nauseous and complained of headaches after aerial spraying at a nearby park.
Whanganui District Council forestry department used a helicopter to spray herbicide in Hylton Park between Brunswick Road and Field Street on Monday morning.
Field St resident Rebecca Briggs says the council should have delivered notices to letterboxes in the area.
"The helicopter was hovering over the park for about 50 minutes and most of us knew nothing about it," she said.
"Even people who had seen the notice in the paper did not know that the spray would be delivered by helicopter."
Council forestry manager Ian Moore said the spraying was done in preparation for replanting in the park which is popular with walkers and cyclists.
"The spray mixture is a combination of glyphosate and metsulfuron," he said.
Glyphosate is the active ingredient in Roundup herbicide used to kill broadleaf and grasses, and metsulfuron is an organic compound used to control blackberry, gorse and scrub weed.
"The spray concentration was below the recommended level and weather conditions were chosen so the spray did not drift beyond the boundary of the park," Mr Moore said.
Ms Briggs said she could smell the spray and it left a metallic taste in her mouth.
"Other people in the area have said they had headaches and nausea."
Neighbour Insook Lee said she was surprised to see a helicopter spraying herbicide close to her home.
"I knew nothing about it and I think a notice in the letterbox would have been courteous.
"I would not use those sprays in my own garden and I worry how it will affect the insect life in the area."
Ms Briggs walks her dog in the park most days but may not be visiting the park during winter.
"Roundup only becomes safe when it is dry and the ground in the park tends to stay wet all winter."
The council has placed signs on gates at both entrances advising that the park is closed for spraying although they did not include the information that the park re-opens on Thursday, June 1.
Mr Moore said the spraying was carried out in accordance Resource Management Act rules.
Pine trees that grew in Hylton Park were harvested in March last year and the replanting will begin in about six weeks.