Tomato prices are expected to increase after more than 5 million plants were poisoned at two north Queensland properties.

Police say it appears a herbicide was injected into the irrigation system at Bowen.

It is the third time crops in the region have been poisoned.

Queensland Health says a batch of tomatoes from the property at the centre of a herbicide contamination investigation is safe for consumption.

The department has tested a batch of the truss tomatoes and no residue of herbicide has been detected.

Queensland Health spokesman Rod Miles says testing of other batches will continue.

But Whitsunday Mayor Mike Brunker says it could cost the local economy $50 million.

"Personally I think that could be a conservative figure," he said.

"No-one's actually sat down and worked that out yet because we're all obviously all focussed on trying to help the police to get a resolution to this case.

"But once it flows on to people not having the money to spend in supermarkets and shops it could have a huge effect."

Cr Brunker says the State Government should offer a reward to help catch those responsible for poisoning the crops.

"This time of year in winter, we supply about 80 per cent of Australia's tomatoes," he said.

"People in Brisbane and all over Australia will be paying a higher price for tomatoes.

"We've got farmers that can't get enough seedlings to replant - that means prices will go up, so people will be paying higher for tomatoes."