Australian police have begun investigations into the poisoning of seven million vegetable plants at a seedling nursery.

The suspected sabotage is estimated to cost $20.3m and could triple the price of vegetables across the country due to a possible shortage.

The incident in northern Queensland state affected four million tomato seedlings, as well as bell peppers, melons and eggplant.

Early investigations showed the irrigation system of the Supa Seedlings nursery in Bowen had been contaminated by herbicide.

Dave Miles, the acting police inspector for Townsville, said workers began noticing the wilting and dying plants in late June, and police were considering a range of motives.

'Possible grudge'

"It could be a grudge, it could be competition based, it could be the result of time-established market share, or it could be an act of vandalism," he said.

Miles added that a dozen detectives were working on the case to determine possible links with three previous poisonings since 2002.

The Bowen region grows the majority of Australia's winter vegetables.

Denise Kreymborg of the Bowen District Growers' Association was quoted by Sky News Australia as saying that the poisoning affected 350 hectares of production land with the potential to produce about 200 tonnes of fresh produce.

She said about 30 growers would be affected.

Kreymborg said growers will continue harvesting their established crops in the next two months, with prices likely to spike around September when the lost seedlings would have been on the market.

"You can expect prices to double or even triple, we don't know for sure," she said. "There's still going to be tomatoes, capsicum, melons, zucchinis and eggplants grown in this area, just not as much."