Fresh produce and food have been a top category at the West Auckland warehouse, with Madagascar crabs and rotisserie chickens flying out the door.
Costco New Zealand country manager Patrick Noone said they were selling around 2000 hot chickens a day and have had to air freight more from the South Island to keep up with demand, while being forced to limit sales to one chicken per customer.
Confectionery has also been a big hit.
"From pork floss, different kinds of chips that we have, to chocolates, sugar candies, everything is selling," Noone told Focus. "There's a real sweet tooth here in New Zealand.
"Meats, bakery does well, the big muffins we sell have done exceptionally well.
"We have a lot of inventory in the building and we have a lot flowing in but our sales have exceeded those expectations.
"We're really trying to re-stock as fast as we can right now and bringing stuff in from the Tasman, offshore like the US and Europe and Asia."
Noone added their Tyre Shop had also been out-performing compared to their Australian locations.
But there were some items yet to be purchased.
"We haven't sold any coffins yet," Noone said. "...which is probably lucky for New Zealanders. We're still marketing them and continue to sell them. One day we'll sell a coffin.
"We [also] haven't sold the $300,000 ring yet but we're working on it."
The store was seeing between 5000 and 6000 daily customers and on average 7000 new members signing up a week.
"Compared to the Australian locations, this is the busiest location we have," he said.
"It would be in the top 20 around the world in terms of transactions."
The "extraordinary" reception has promoted the US retailer to start working on plans for stores in Christchurch and Wellington.
"It's very encouraging for further investment in New Zealand in the future," he said.
"We've got some sites we're looking at and hopefully something to announce in the near future."
