Fergusson said the store also had electric blankets in stock, which had been selling well.
Paul Wilkey, owner of Tauranga Firewood, said the phone had been ringing more since temperatures had dropped.
Wilkey said the company supplied wood to customers all year round so orders had been steady and he was yet to run out of wood.
Trade Me spokeswoman Millie Silvester said 115,000 searches for heaters were made on online auction site Trade Me last week.
These searches were up 50 per cent on the same time last month, making up nearly half of all searches for heaters in May.
"We've seen a staggering number of searches for heaters, merino clothing and electric blankets," Silvester said.
Searches for electric blankets hit nearly 10,000 in the past week, up 27 per cent on the same time last year, 41,000 searches were made for dehumidifiers and 51,000 for merino clothing.
At The Warehouse, trends were the same.
Merchandise manager Jenny Epke said the cold blast has seen winter trading surge in the past week where electric blanket sales rose 132 per cent.
"'Heaters' was the top-searched term on thewarehouse.co.nz and store sales of electric heaters were up 126 per cent," Epke said.
Sales of children's puffer jackets doubled and umbrella sales rose 72 per cent.
Men's and women's merino knitwear sales were up 39 per cent on the same time last year.
Epke said she had also noticed an increase in sales of DVDs, toys, blankets and cushions, suggesting households were snuggling up with home entertainment for cold nights.
Countdown merchandise manager Steve Mills said sales for heaters, firelighters, hot-water bottles, blankets, thermal socks, slippers and hosiery had doubled in the past 10 days.
"As we get shorter days and much colder nights, we all want to stay home, keep warm and we start to crave comfort food.
"We've seen more Kiwis grabbing roasts for dinner, slow-cooked meats, winter vegetables and lots of soups."
- Additional reporting NZ Herald
Some of Trade Me's quirky listings to keep warm this winter:
• A USB cup warmer to plug into your computer, keeping your cup of tea warmer for longer.
• Unicorn heated slippers which heat up when charged at a USB port.
• A heated throw rug, for those who feel too fancy for an ordinary electric blanket.
• Toe warmers for sticking to each individual toe, warming them for up to five hours.