An independent group called CTAG is in charge of setting New Zealand's terror alert level

Prime Minister Jacinda Ardern says it is not known at this stage how long New Zealand's terror alert will remain on high. Photo / Mark Mitchell

Prime Minister Jacinda Ardern says it is not known at this stage how long New Zealand's terror alert will remain on high.

That is because it is not her decision.

On Friday, New Zealand's terror threat level was raised to high for the first time ever.

This means, according to the independent inter-governmental agency responsible for setting New Zealand's terror threat: "[A]terrorist attack, violent protest activity or violent criminal behaviour is assessed as very likely".

The threat level is set by a group called CTAG – Combine Threat Assessment Group – which is made up of multiple government agencies.

These are the NZ Security Intelligence Service (NZSIS), the Government Communications Security Bureau (GCSB), the Defence Force, Police, Corrections, Ministry of Foreign Affairs and the Civil Aviation Authority.

CTAG was set up in 2004 in response to changes in the global terrorism environment post-September 11, 2001, and the Bali terror attacks in 2002.

The Prime Minister confirmed on Saturday that this was the first time New Zealand's terror alert has been moved to high.

In fact, it's the first time it has been moved any higher than low and has never been at medium.

In 2014, New Zealand's threat level was moved from very low, to low after Cabinet approved terms of reference for a review of settings in relation to "foreign terrorist fighters".

CTAG has a six-tiered threat level system starting at negligible – where a terrorist attack, violent protest activity or violent criminal behaviour is assessed to be "very unlikely".

It goes all the way up to extreme, which is assessed as when a terrorist attack, violent protest activity or violent criminal behaviour is expected imminently.

CTAG is tasked with setting the threat level, and it does so independently.

But how New Zealand responds to that threat level is determined by the senior officials' advisory group for emergency situations – ODESC.

It considers the appropriate response and then makes recommendations to senior ministers, including the Prime Minister.

All public sector agencies, including those represented by CTAG, are responsible for responding to the threat level.

For example, the police have decided to station armed officers across the country.

Police Commissioner Mike Bush said Kiwis could expect to see a "highly visible" police presence on the streets around schools, businesses and "even in the air".

He said this was to help make people feel safe to go about their daily lives.

Although the police have not explicitly said so, it is thought the increased police presence was in response to the threat of any copycat or retaliatory attacks.

Events across the country with large gatherings of people have been cancelled in the wake of Friday's shooting.

CTAG continuously reviews the threat level and uses a wide range of intelligence and information to analyse the intent and capability of potential terrorist actors.

Other countries, such as the UK, Australia and Canada, have groups similar to CTAG.