COVID-19 trans-Tasman bubble: How the 'traffic light' system works and what happens in an outbreak?

Quarantine-free travel between Australia and New Zealand will open on April 19, and be managed by a 'traffic light' system.

But what is the traffic light system and how will it work?

Like New Zealand's four-tiered lockdown alert system, the traffic light system will allow different responses and restrictions depending on how many cases of COVID-19 each end of the travel door is experiencing.

When it's green, COVID-19 cases are at low risk of transmission - for example, any cases that do emerge are linked to the border.

In this stage flights to and from Australia are "likely" to continue.

At orange, the chances of an outbreak is heightened - an unknown COVID-19 case has emerged and although it's "most likely" linked to the border, there's no way to be sure.

At this stage, quarantine-free travel from affected states would be paused for up to 72 hours.

At red - there are multiple cases of unknown source and the state would enter a longer period of lockdown.

Here, flights would be suspended for an extended period.

At both orange and red, people returning to New Zealand may be required to get a COVID-19 test prior to departure, and self-isolate upon their return.

The system
The system Photo credit: Supplied

If they're returning from a state where travel has been paused, they could be put into managed isolation.

Masks must be worn on all flights and anyone who has tested positive for the virus in the last 14 days will not be allowed to travel - nor will anyone awaiting test results. 

Prime Minister Jacinda Ardern said on Tuesday all travel will be undertaken at "flyer's risk" as either side of the bubble could pause travel with little warning at any time.