Air New Zealand's $9 airfares have sold out within minutes this morning.

Travellers struggled to access the airline's Grabaseat website after the airline placed 1000 $9 flights on 32 domestic routes from 9am.

And within the hour, the sold-out label was posted across the site, although $49 fares remained in play for key destinations, including Auckland-Wellington.

The slashed airfares come three days after the airline cut fares across the Tasman to as low as $69 in a bid to fill seats and just hours after it halved the number of flights to Samoa.

Chief revenue officer Cam Wallace said the $9 domestic fares were in response to softening demand as a result of coronavirus.

"On Monday we'll be offering these absolutely outstanding one-way fares to all of our 20 domestic destinations," he said.

"With deals this good, savvy Kiwis will be able to book a relaxing weekend away, catch some autumn sun, or take the kids away to explore a new part of the country."

On Friday Wallace said customers could get some "ridiculously good deals" on flights across the Tasman, citing one-way fares from as $69 on flights from Auckland to Melbourne as an example.

"Like all airlines we have seen some softness in demand on routes like the Tasman where we now have some empty seats due to travellers mainly from Asian destinations not connecting between New Zealand and Australia.

"Kiwis and Aussies are going to be able to head away for holidays or to see family and friends or do business for cheaper than a night's accommodation at most hotels.

Air New Zealand, Qantas, Jetstar and Virgin Australia have all reduced capacity over the Tasman as a result of coronavirus travel bans and fears over the outbreak but are dropping prices to ensure planes are as full as possible.

Earlier on Sunday, Air NZ announced it was halving its flights to Samoa after the island nation insisted that all passengers carry a medical certificate confirming they are fit to travel.

The national airline has cancelled one flight, and reduced its service from six flights to three from Monday after being ordered to do so by the Samoan Government from midnight Sunday.

Samoa's health officials have also introduced a requirement for all passengers entering the country to carry a medical certificate dated within three days of travel in response to the global coronavirus outbreak.

"All passengers to Samoa are now required to carry with them a medical certificate, indicating they are well and able to travel," Air NZ said.

The airline has cancelled its Auckland-Apia flights scheduled for today, Friday and Sunday.