The government's new Covid-19 tracing app is now available to download - a day earlier than its official launch.

The NZ Covid Tracer app was supposed to be launched tomorrow, but some people have been able to download it early from the Apple Store.

In a statement, a Ministry of Health spokesperson said the app will be formally launched tomorrow.

"As part of preparations for the go live of the contract tracing app it was submitted to the Google and Apple stores for processing this evening.

"There can be a wide variation in times between submitting apps and them going live."

Prime Minister Jacinda Ardern has described the mobile app as a "digital diary".

Once downloaded, the app asks users to create a personal account including their name, email address and phone number.

Users are then asked to scan QR codes at businesses, public buildings and other organisations to keep a record of where they have been, in the event they need to reached for coronavirus contact tracing.

The information will be stored for 31 days by the Ministry of Health, which uses Amazon Web Services in Sydney to store the data, before being automatically deleted.