Just over 24 hours before New Zealand is due to enter full lockdown, 40 new cases of coronavirus have been confirmed, bring the country’s total to 155.

Four of the cases were confirmed as contracted through community transmission and six people were in hospital, but none in intensive care, said the director general of health, Dr Ashley Bloomfield.

The increase in cases came as New Zealand prepared to enter a month-long lockdown and panic buying occurred across the country. Following the prime minister’s announcement on Monday New Zealanders bought enough food to feed 10 million people, the Countdown supermarket said, despite being a nation of fewer than 5 million.

Police were called in to break up fights at supermarkets, despite the prime minister repeatedly calling for “kindness” and calm. Long lines formed outside gun and hunting shops, with some New Zealanders – even in urban areas – appearing to stockpile weapons and ammunition.

Newshub (@NewshubNZ) Queues outside gun shops as Kiwis panic-buy firearms before lockdown https://t.co/e06QsQxm1J pic.twitter.com/5D0EAUq3c2

The national carrier Air New Zealand put on more domestic flights and bigger planes to get people home, especially university students not wanting to enter month-long seclusion in dormitories.

The department of foreign affairs said it may now be too late for tens of thousands of Kiwis overseas to come home, as flights stopped around the globe, and they should now prepare to “shelter in place”.

Mercy flights for Kiwis stranded overseas are being discussed, but have not been confirmed, the prime minister said, and that option was a logistical nightmare with hubs such as Singapore now closed even to those in transit.

Jacinda Ardern said she knew what she was asking the population to do was “huge” but it was essential to save lives and avoid the healthcare system being overwhelmed, as in Italy and Spain.

She said national self-isolation measures meant exactly that; stay home, and only go out for essential supplies. Physical contact with anyone not in your own household should cease, Ardern said, though couples who shared custody of children in different homes could see each other, and people living alone could form buddy systems with one other individual nearby to stave off lonliness, as long as it was an exclusive “bubble” arrangement and “you stay faithful to them and they stay faithful to you”.

“If community transmission takes off in New Zealand the number of cases will double every five days. If that happens unchecked our health system will be inundated and tens of thousands of New Zealanders will die,” Ardern said.

“There is no easy way to say that – but it is the reality we have seen overseas – and the possibility we must now face here. Together we must stop that happening, and we can.”

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As New Zealanders prepared to self-isolate there was widespread confusion about what qualified as an “essential service” that would be allowed to stay open. The Warehouse, a discount general goods store, announced it would remain open, though the prime minister later said this would not be permitted.

Questions were raised about whether butchers and bakers would be allowed to remain open in addition to supermarket chains, and if liquor stores would be classified as essential.

At a rowdy press conference the Covid-19 taskforce was unable to answer many questions relating to what counted as an essential service but said it would have more information by Tuesday night or Wednesday morning.

Finance minister Grant Robertson has announced a six month principal and interest payment holiday for mortgage holders, which has been welcomed.

But no similar arrangement has been made for renters, worrying many who have lost their jobs, or had their hours cut back.

“A six-month mortgage holiday for people whose incomes have been affected by COVID-19 will mean people won’t lose their homes as a result of the economic disruption caused by this virus,” Robertson said.

• This headline of this article was amended on 25 March 2020. A rise from 115 to 155 cases is an increase of just under 35%, not “nearly 50%” as an earlier version said. This has been corrected.