Australian police said today that they were investigating a mysterious mass poisoning of seven million tomato plants and other crops which cost tens of millions of dollars in damage and is expected to send prices soaring.

Detectives were investigating whether vandals or a competitor with a grudge had put herbicide in sprinklers at a major nursery near the northeastern city of Cairns, wiping out 16 million tonnes of produce, mostly tomatoes.

"It could be a grudge, it could be competition-based... or it could be an act of vandalism by a couple of young hoons (vandals) -- we can't rule that out either," said Townsville detective Dave Miles.

The damaged crops also included capsicums, pumpkins, melons, aubergines and courgettes due to be harvested and transported across Australia and to New Zealand and Vanuatu in the coming months.

Prices are expected to double or triple as a result, and growers' profits will suffer a major hit.

"This sabotage has been estimated as potentially costing the industry and the wider community up to 50 million dollars (42 million US)," said national Agriculture Minister Tony Burke, adding that he was "shocked".

It is the fourth and most dramatic incident of crop sabotage in the area in the past decade, and police said they were contemplating a reward for information.

Queensland Police Minister Neil Roberts said he would consider any request put forward by the police commissioner for a government reward.

Produce farmers have condemned the crime as an act of "industrial sabotage".

"That's the view of the police -- that it's been deliberately sabotaged," growers' association spokesman Carl Walker told the Australian Broadcasting Corporation."