Oysters are dying on Rick Yorke's Ohiwa Harbour farm and he fears he may lose up to 70 per cent of his stock.
"It's not just spat under 12 months that are dying, but older oysters as well. I estimate I could lose between 50 and 70 per cent of my oysters - I'm hoping for the lower figure," said Mr Yorke who took back management of the farm near Whakatane he had leased in 2007, turning around its performance with plans to harvest up to 60,000 dozen oysters a year.
"This year we were looking at our best season so far, until this hit."
The Ohiwa Oyster Farm is one of dozens throughout the upper North Island stuck by what is believed to be a virus which is killing the shellfish.
"Every year, we get some deaths but nothing like this. It went through the farm about three weeks ago and I'm hoping the worst is over," said Mr Yorke.
MAF Biosecurity New Zealand has said the healthy shellfish are safe to eat and, at the popular Ohiwa Oyster Farm roadside shop, it's business as usual with people calling in to buy Pacific oysters.
"It's that way for all oyster farmers because we have to sell the oysters now before they spawn." Once oysters spawn they loose weight and can't be sold until they "fatten up" again.
The New Zealand Oyster Industry Association for Aquaculture New Zealand and MAF Biosecurity are working together to investigate the unexplained Pacific oyster deaths.
Aquaculture New Zealand CEO Mike Burrell said since mid November farmers had seen an increase in the mortality rate of young oysters (known as spat) on farms from the Bay of Plenty in the east and from north of Kawhia in the west.
"Typically, mortality rates for cultivated Pacific oysters are between 5 and 10 per cent.
This year the rates on some farms have varied between 30 to 80 per cent which is very concerning for the industry," Mr Burrell said.
MAF has its aquatic disease specialists on the case and has taken 250 samples for analysis to try and determine what is causing the die-back.
Mr Yorke said the impact on his farm was easy to see with open shells among the healthy ones. While it has yet to be determined what the cause of the deaths is, Mr Yorke wonders if heavy rains in spring led to an increase in nutrients washing into harbour, promoting a growth of bacteria in the water.
He's philosophical about the impact on the future of oyster farms. "I liken it to a pukeko crossing the road - some of us will make it and some of us won't. If it happens again next year it might be like trying to cross the motorway in a mobility scooter."