Five people were arrested near Hokitika on the West Coast after a protest against a 1080 operation on Tuesday morning.
Police were called to the farm, south of Hokitika, where a helicopter drop of the pest poison had been due to start.
They say nails had been laid on the road, puncturing the tyres of a police car and a private security car.
A group of protesters had also got through a cordon and were in the operational area near the helicopters.
Four men and one woman have been charged with offences under the Trespass Act and the Biosecurity Act.
Animal Health Board spokesman Nick Hancox says the aerial operation got under way after a delay and would continue on Wednesday.