This is where New Zealand gets its guidance in our darkest days.

Pronounced "O-desk", the acronym is actually Odesc. It is a Top Secret committee which forms at times of national crisis.

The letters Odesc stand for the formal-sounding Officials Committee for Domestic and External Security Coordination.

It is made up of chief executives of those agencies you might expect respond to incidents such as the shootings in Christchurch, the city's earthquakes, the Pike River mine disaster or the Rena oil spill.

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The difference is, when an urgent Odesc meeting is formed, those individual chief executives don't stand alone.

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Years of expertise and training converges - quickly - in the high-security bunker at the Beehive to bend their collective intelligence and will to the safety and security of New Zealand.

It did so on Friday in the immediate aftermath of the Christchurch shootings and continues to do so, making decisions of national significance which shape our response to a crisis and how we, as a nation, will emerge.

"They drop whatever they are doing. When the whistle goes up, everybody runs," says a former senior public servant who has been present at a number of Odesc meetings.

Such an alert pulls in police commissioner Mike Bush, NZ Security Intelligence Service director general Rebecca Kitteridge, Government Communications Security Bureau director general Andrew Hampton, NZ Defence Force chief of defence Air Marshal Kevin Short, State Services Commissioner Peter Hughes, Ministry of Foreign Affairs and Trade chief executive Chris Seed, among others.

All are within walking distance of the Beehive - Wellington's close public service hub serves Odesc well for gathering quickly.

A family laying flowers at the Linwood mosque. Photo / Mark Mitchell
A family laying flowers at the Linwood mosque. Photo / Mark Mitchell

"Bringing seven or eight chief executives together has great value. It also has costs because these are very busy people," says the former public servant. "You don't have time to sit around.

"You have very little time to make these decisions. It's not for the faint-hearted."

It means for the system to work, meetings are kept as short as necessary or run as long as needed with the guiding principle to extract as much value as possible before returning to their agencies.

The Beehive bunker, where the meetings take place, is called the National Crisis Management Centre. It is the nerve centre of government, with secure communications facilities and protection from external surveillance.

The meetings are chaired by the chief executive of the Department of Prime Minister and Cabinet, currently Brooke Barrington.

DPMC's job is to be the nexus of the public service and to connect the Prime Minister's office - Ardern's ninth floor staff - to the rest of government.

Having gathered in the bunker, the chief executive of the agency at the forefront of the response goes on to lead the meetings. In this case, the task falls to Bush.

"One person will be the lead agency and the rest of us will do all we can to help the lead agency get the best out of it for New Zealand.

"Everything Mike Bush has done in his career has been brought to bear in what he has been doing in the last few days."

Police commissioner Mike Bush briefing media on the Christchurch shootings. Photo / Mark Mitchell
Police commissioner Mike Bush briefing media on the Christchurch shootings. Photo / Mark Mitchell

The situations with which Odesc grapples are not all unfamiliar to those on the committee. There are constant exercises to develop response strategies - biosecurity, environmental disasters, mass arrivals of refugees, pandemics and terrorism have all been gamed out, including role-playing simulated events.

"You spend your whole time preparing for something you hope won't happen."

For all the preparation, the extraordinary events for which Odesc convenes are usually those which - even if planned - have not actually taken place.

"This is a real terrorist incident. This will be a first."

Those gathered in the meeting room are supported by a phalanx of officials who wait outside. This is the real, specialised expertise on which Odesc members rely.

Inside, the seriousness of events is "palpable". There is a clear realisation of the sheer weight of responsibility - members know their presence at the meeting signals a time of crisis.

They are clear the national security of New Zealand rests with them, and the decisions they make in that room.

The first hours are marked by a void of information into which facts fall and contrive to form pictures. "Intelligence is absolutely critical."

Each crisis throws up unexpected challenges. "There are always things coming out that you haven't imagined."

In the current situation, the livestream of the attack on Facebook was likely an aspect of the event which defied best preparation.

Decisions are made, plans are formed. There's no time for second-guessing or checking operational steps with elected officials.

The former public servant spoke to the Herald to give people an understanding of how the Government responds in times of crisis, and with a desire to say nothing which would impact on the critical work Odesc is currently carrying out.

"It's a very practical expression of the way our democracy works in New Zealand.

"The fact it is not that well known in the public domain does not matter. It's a very good thing for ministers and Prime Ministers to have everyone around that table and working as one."

This morning, Barrington and Bush were expected to brief Cabinet - to give our elected leaders the best advice our entire Government has to offer on what has happened and what will happen next.

And when Ardern speaks to the nation, she will do so as a Prime Minister informed and supported by the best our public service can offer.