New Zealand has recorded its biggest single-day increase of Covid-19 cases, with 11 new positive tests, taking the country’s total to 39.

Health officials said there were no cases of community transmission, and have urged residents to self-isolate judiciously if they fear they are a risk to the community – advice that most people are complying with.

New Zealand is yet to endure life-threatening cases of the virus or any deaths, and director-general of health Dr Ashley Bloomfield said now was the time for residents to be “nice and kind to one another”, to prevent panic or mental strain.

Only one person currently required professional care for the virus; an elderly man who was being treated in Queenstown’s Lakes District hospital.

A fresh case in Hawke’s Bay meant that all regions bar the South Island’s west coast had recorded cases of the illness, all arising from overseas travel, authorities said.

The number of cases has jumped in line with a huge surge of testing. Until Monday, only 524 tests had been conducted during the entire outbreak.

This week, health officials increased testing rates dramatically to around 1,000 a day, with more testing centres being opened in the major centres, especially Auckland.

The government expects the overall number to continue to grow as tens of thousands of New Zealanders return home, with Air New Zealand in talks with the foreign minister to charter mercy flights for those stuck overseas.

Like Australia, New Zealand closed its borders to non-residents and non-citizens on Thursday, with Prime Minister Jacinda Ardern to consider further measures as required.

So far schools, shops and most workplaces remain open throughout the country, though some experts view this as a mistake, and say the country should be taking more urgent measures to contain the virus, which has led to thousands of deaths overseas.

In Auckland, which has seen the largest number of cases, at 17, the council on Friday closed libraries, pools and recreation centres for the next fortnight, while Wellington banned cash on public transport.

Meanwhile Air New Zealand has been granted a NZ$900m (£445m) loan by the government, after slashing 30% of its 12,500-strong workforce and 85% of its long-haul routes due to plummeting demand. Flights run by the national carrier dropped from 3,600 in an average week to fewer than 1,500 this week.

In an unprecedented move, the major supermarkets joined forces on Friday to urge Kiwis to “stop panic buying”, saying there would be enough food for everybody if people returned to shopping in a normal way rather than hoarding and stockpiling.

Supermarket Countdown’s managing director, Natalie Davis, said in a statement: “We are not running out of food. However, we simply cannot get it through the network fast enough if Kiwis don’t slow down their shopping.”

Supermarkets said the country was in a favourable position because it produced much of its basic food supplies domestically, including fruit and vegetables, meat, bread and dairy.

There are also reports from garden centres of panic buying vegetable seedlings and gardening equipment.

A catch-all email address – nhccselfisolation@health.govt.nz – has been set up for anyone concerned about the illness, with experts ready to respond with advice from health, police and immigration.