Experts are divided over whether people should sanitise packages or deliveries received in the mail during the COVID-19 crisis.

As of Wednesday at 11:59pm, the Government's coronavirus response was escalated to alert level 4, requiring all Kiwis to self-isolate in their houses for four weeks.

This lockdown is designed to put the brakes on the COVID-19 pandemic in New Zealand, as it stops people coming into contact with one another and transmitting the disease.

But some experts believe mail and package deliveries during the lockdown could be another method for it to spread, as a coronavirus carrier could transfer the virus if it sticks to the mail they send, only to be touched by the recipient at the other end.

Speaking on NBC TV show Today, medical expert Dr John Torres said the coronavirus may have the capability to live on cardboard boxes for up to 24 hours and plastic packaging for as much as 72 hours.

"[It can survive on] anywhere from copper, around four hours, to stainless steel and plastic, two to three days," he said.

"The guidance from WHO is that the likelihood of an infected person contaminating commercial goods is low.

"Cardboard is right down the middle - 24 hours is how long it could live on there and still be what we call viable, meaning it could still pass on coronavirus."

Dr Torres' claims were a reference to a paper published in the New England Journal of Medicine, which found that the SARS-CoV-2 virus survives three times longer than the original SARS.

However, not all experts are in agreement on the dangers of mail delivery during a pandemic.

In the US, for instance, the United Parcel Service (UPS) has been forced to defend its decision not to take additional sanitising precautions.

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In defending itself, it cited both the World Health Organisation (WHO) and the Center for Disease Control as saying that transmission through packages is low-risk.

"There is currently no evidence that COVID-19 is being spread through the mail," a UPS spokesperson told The New York Times.

"The guidance from WHO is that the likelihood of an infected person contaminating commercial goods is low.

Dr Torres' claims were a reference to a paper published in the New England Journal of Medicine, which found that the SARS-CoV-2 virus survives three times longer than the original SARS

"The risk of catching the virus that causes COVID-19 from a package that has been moved, travelled, and exposed to different conditions and temperature is also low."

This is because coronavirus is predominantly spread through droplet infection, while transmission via packages and letters requires what's called 'smear infection', the German Federal Institute for Risk Assessment says.

Indeed it's believed the greatest threat to wellbeing actually lies with couriers and postal workers, as they come into contact with packages hours, rather than days, after they're sent.