Supermarkets are "scrambling" to keep their shelves stocked and their doors open as they face an 80 per cent increase in business since the start of the national lockdown.

And the head of one of New Zealand's major supermarket companies supports butchers - and other food supply businesses - opening during level 4 restrictions if there is a firm safety strategy in place.

Foodstuffs North Island chief executive Chris Quin told Newshub this morning that more that a million cartons of goods were delivered to its stores last week.

Foodstuffs owns New World, Pak'nSave, Pams and Four Square,

Quin said the priority was to keep staff and customers safe - but also to keep the shelves as full as possible.

However, with so many staff forced into isolation due to the pandemic, it was hard at times.

"There will be times - an hour or two here or there - where shelves won't have every brand and every product," he warned.

"We are definitely under pressure… but the global supply chain been under pressure all year.

"We've got stock as long as people shop normally - if we do that we'll be able to keep filling the shelves and keep this as calm as we can."

Quin rejected suggestions of price gouging, insisting Foodstuffs were keeping specials and promotions running where possible.

"We have to make sure we have stock on items being promoted," he said.

"We start tracking straight away [when a lockdown starts] to make sure prices are kept even."

Quin said it was hard to know what the increase in profit was for Foodstuffs.

"There's no doubt volume has been up, but so has a lot of cost."

Costs included hundreds of staff in isolation unable to work.

"We are scrambling hourly to make sure shops stay open," he said.

Quin supported the call from butchers yesterday to be able to operate and supply stock to consumers.

"I think we'd like everybody back in business," he said.

"It's good for us to have competition and good for us to have the market trading normally.

"Fundamentally [butchers] probably could operate safely - but the question is, what is the strategy about the number of places people are going to?"

On Sunday a supply glitch saw a massive meat shortage at Countdown stores - leading to butchers calling on the Government to allow them to run click-and-collect.

With smaller greengrocers, bakeries and butchers restricted to deliveries only at alert level 4, supermarkets have been dealing with high demand.

The workers have been widely praised but - perhaps due to the stress of lockdown and additional mask requirements - tension has run high for some customers.

One man was arrested for spitting at a Countdown staff member after being refused entry to a store in Christchurch while other customers have vented at a lack of meat on the shelves.

Countdown spokeswoman Kiri Hannifin told Morning Report yesterday that since the mask mandate started, staff had dealt with about 20 threatening incidents a day, but people were mostly doing the right thing.

"By and large people are very, very good and doing the right thing ... if it gets violent or aggressive or too difficult for us we call the police."

Hannifin said yesterday that more than 1000 of the supermarket chain's staff are isolating - some also because they don't have childcare available or are vulnerable to the virus.

"We're expecting this number will continue to increase, and we're managing this as best as we can. The safety of our team and customers is our top priority."

Meanwhile, Foodstuffs said about 350 of its staff were isolating, but that number too was expected to rise as Pak'nSave Clendon and Manukau have recently been added to the locations of interest list.

Some supermarkets, including Countdown Birkenhead, Albert St Metro, Hauraki Corner, Māngere Mall and Halsey St Metro, have been temporarily shut due to low staffing.

All Foodstuffs stores remain open, but opening hours may be reduced when a store becomes a location of interest.

"Customers should check the website or Facebook page of the New World, Pak'nSave or Four Square for the latest information," said spokeswoman Antoinette Laird.