He predicts that vegetables such as broccoli, cauliflower and lettuce could soar from average summer prices of about $1.99 a head to $3.99 in winter.
Avocados may rocket from 99c each to $3.99, and capsicums may go up from $1.99 to $3.99 each.
Mushrooms are likely to rise from $4.99 a packet to $6.99 and peas may shoot from $1.99 a kilo to $2.99.
But Forsyth says sky-high tomatoes look set to fall to about $8.99 a kilo in the coming months as more demand is generated.
Winter staples such as potatoes, carrots, parsnips, swedes and leeks will all still be very good value.
Fruit buyers are likely to fare better, with little movement expected in the price of apples, kiwifruit, lemons, grapefruit and tamarillos.
Oranges may drop from $3.99 a kilo in July to $1.99 in August.
"Prices of some fruit we import all year round - like bananas, grapes and mangos - tend not to change much during winter," Forsyth says.
He believes a ceiling exists on the price of most fruit and veges and that it is set by consumers.
"Sellers know that, for instance, when broccoli and cauliflower reaches $3.99 people will not be prepared to pay any more than that," Forsyth says.
"That is also when you start seeing half broccoli and half cauliflowers for sale as people tend to buy a little less when the cost becomes too high.
"Taking a little time to shop around for both in-season and out-of-season produce and checking in-store specials is still the best way to keep the grocery bills down while keeping the quality up."
Pair fed up with rises
Labourers Michael McKeever and Marcel Brennan have stopped buying fresh vegetables because of high winter prices.
The men, originally from Ireland, say they are shocked at the cost of the likes of lettuce, broccoli and avocados.
They usually shop at the Countdown supermarket in downtown Auckland.
"I'd love to have veges in winter but I'm not paying $2.59 for an avocado," McKeever, 29, says. "It is also $2.79 for a lettuce and broccoli is $2.49 each, which is way too much. It's a joke that some of these items will rise to about $4."
Brennan, 22, looks for cheap packets of soup for a vege fix during winter. "The price of fresh vegetables and some fruits in New Zealand at this time of year is a disgrace," he says. "It is much more expensive here than it is in Australia or Ireland. I don't know how these prices are justified."
