(Stuff) - 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. 

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.