Supermarket staff are being abused by frustrated members of the public as stores struggle to keep up with the impact of coronavirus.

As of Wednesday afternoon, New Zealand had 20 confirmed cases of Covid-19 - an increase of eight from Tuesday.

Meanwhile, at supermarkets, products like hand sanitiser and toilet paper had been flying off the shelves since New Zealand had it's first case confirmed.

A woman who attended New World Kumeu recently said customers were being revolting to its checkout operators.

Queuing for the checkout, the woman witnessed customers give staff a "really hard time" about products being out of stock or having limits on them.

"The checkout operator said that they're getting abused frequently at the moment by frustrated customers," the woman posted.

"This is pretty shabby behaviour. How about we all leave our entitlement and self-importance at the door and show a little patience and understanding?"

Shoppers were asked to be kind to Foodstuffs' (New World and Pakn'Save) staff on the front line who were trying to do their jobs.

"We may all need to take a deep breath from time to time, everyone is doing their best," head of corporate affairs Antoinette Laird said.

"The arrival of Covid-19 has absolutely put pressure on our supply chain, but it's robust and we're coping well under the circumstances.

"The best thing people can do to help us get through this to shop normally and resist the temptation to stock up, this is where the pressure comes."

Elsewhere, Countdown supermarket reminded Kiwis there was no need to panic-buy.

New Zealand's first confirmed case of Covid-19 was announced on Friday, February 28, and the next day spending at chemists and supermarkets skyrocketed.

Paymark data showed spending up 75.5 per cent in Auckland compared to the equivalent day last year - the city's supermarket spend shot up 40.2 per cent.

People would miss out on products if others bought more than they needed, Countdown general manager of health and safety Kiri Hannifin said.

"We'd strongly encourage customers to shop as they normally would, this includes shopping in-store if you don't need to shop online, as well as being thoughtful of others to ensure that all New Zealanders have access to the food and products they need," she said.

"This is a constantly changing situation, which we'll of course continue to monitor, but the main message to New Zealanders is that there is no need to panic about food and groceries.

"We hope that our customers and all Kiwis are taking heed of our Prime Minister's words and being kind and taking care of each other."