A contractor has sprayed seven Marlborough vineyards with the wrong chemical, costing the grapegrowers around $1.6 million in lost revenue, a viticulturist says.

The affected vineyards, about 5 kilometres from Blenheim in the Wairau Plain, were sprayed using a fertiliser that makes the wine produced unfit for export.

Consultant viticulturist Murray Paterson, who was brought in to advise one of the growers, said the phosphorus and potassium-based fertiliser, Perk Supa, was used by the contractor in March.

The product was flagged by New Zealand Winegrowers and not recommended for use because it left a residue that prevented wine being exported to key markets, such as Canada.

READ MORE:

* Compo over vineyard spray

* Winemakers taste the organic reward

* Rogue spray ruins vines

This meant the crop was unlikely to be bought by wine companies, unless they were making wine purely for the New Zealand market, as the threshold for the residue was higher.

Even then, the grapes would be bought at a much lower price and Paterson said the growers would only be able to recover some of their operating costs.

"The mistake will cost the growers their income for the next two years," he said.

"It's not just the loss of the crops but how much they've spent, it costs around $8000 per hectare to bring grapes to harvest."

The market value of the 900 tonnes of grapes before they were contaminated would have been around$1.6 million, a cost the growers would be trying to get back from the contractor, he said.

He said it was likely the man would have some kind of liability insurance, which could be used to pay back the growers.

The contractor said he would not comment as there was an ongoing process to resolve the issue.

Paterson said the man would not have acted deliberately but he must not have checked the status of the fertiliser, which was used more commonly three of four years ago.

He said it was commendable the contractor had been honest and filled out spray diaries detailing the use of the offending product.

If he had stayed quiet the grapes would have gone through to wine companies, Paterson said.

Sustainable Winegrowing New Zealand general manager Philip Manson said he would not comment on individual cases, but said New Zealand Winegrowers' spray schedule listed Perk Supa as a flagged product.

"We've said for some time that the potential residue issue in certain markets means it's best not to apply this product," he said.

Wine Marlborough chairman Clive Jones said it was rare to get sprays wrong.

There was no shortage of information and New Zealand Winegrowers provided clear guidelines about what could be sprayed and when, he said.

It was understood that last year, another vineyard in the Waihopai Valley was mistakenly sprayed with a harmful pesticide.