The government has provided more clarity about what constitutes essential services once the country is in lockdown.

MBIE deputy chief executive Paul Stocks says the purpose of escalating New Zealand's COVID-19 alert level to 4 is to stop the virus in its tracks and reduce contact between people.

"That's why we need as many businesses as possible to close their premises now if our one shot at beating the virus is to be successful.

"Only the businesses absolutely essential to ensure the necessities of life, like supermarkets and pharmacies, can stay open. If in doubt, the business premises should be closed."

He says this means the Warehouse needs to close its stores.

Paul says "big box" retailers like Bunnings, PlaceMakers and Mitre 10 can stay open to trade customers for essential purposes only.

"These retailers play an important part in the construction supply chain, but they cannot sell goods to the general public."

Dairies can continue to operate, but must ensure strict physical distancing rules for customers.

Paul says dairies sold basic food items like bread and milk, often to the elderly who may not be able to get to a supermarket.

"If any dairy breaks the rules, we will shut it down. If there is evidence of systemic abuse, we will remove them from the essential services list."

All restaurants, cafes and bars must close all aspects of their operation, including delivery.

That means food delivery services like Uber Eats and Deliver Easy cannot operate either, however, the delivery of food that is not pre-cooked will be allowed.

"We are doing further work on online ordering of non-food products for home delivery to see if this type of retail can be conducted safely. We will update advice on this once further decisions are made."

Liquor stores will also need to close, except where they operate in licensing trust areas.

Businesses that are a critical part of the supply chain for essential services are also able to continue operating, but must do so in a way that is safe.

"For example, if you make chemicals that are needed for our wastewater plants, then we need you to keep operating at the minimum level required."

Paul says there will be significant limits on what people could buy, however, the list could be adapted if it is found some essential services needed to be made available.

Further details on essential services:

• Dairies to stay open, with "one-in one-out" rule
• Food delivery prohibited, except meals on wheels and delivery of food not pre-cooked
• Liquor stores closed, unless within a Licensing Trust Area and with "one-in one-out" rule
• Self-service laundries can stay open, two-metre physical distancing to be enforced
• Retirement villages included as an essential service
• The Warehouse to close stores to the public
• Bunnings, PlaceMakers, Mitre 10 and other retailers essential to the supply chain for building and construction can stay open for trade customers for essential purposes only
• The Tiwai Point smelter exempt from closure
• NZ Steel shut down in a way that allows for production to recommence easily
• Pulp and paper plants to shut down non-essential elements in a way that allows for production to recommence easily while maintaining essential production
• Methanex to remain in production, but at a scale consistent with stability of gas supply.