The police officers who rushed to help when 49 people were shot and killed at two Christchurch mosques today are professionals, the boss of their union says.

But that doesn't mean they won't be affected by the horror that occurred on an otherwise normal Friday afternoon, Police Association president Chris Cahill said.

"I don't think anything can prepare you for something like this. It's horrific ... for nearly all the officers it will be the biggest and toughest thing they've had to deal with in their career."

It's only eight years since a 6.3 magnitude earthquake took 185 lives and changed the face of large swathes of Christchurch, and today's massacres will bring back the trauma of the quake for some officers, Cahill said.

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"We'll have to watch for that."

Cahill, a police officer of 30 years, flew to Christchurch this afternoon to begin the 12,500 member association's work in supporting those on the beat.

Police Association president Chris Cahill has already flown to Christchurch to support police officers following today's massacre at two mosques in the city. Photo / File
Police Association president Chris Cahill has already flown to Christchurch to support police officers following today's massacre at two mosques in the city. Photo / File

The police, like the society they reflected, had gotten better at supporting people through trauma.

The key officers involved, such as those involved in working with those dead and injured, were required to have counselling following the event, he said.

Getting in and helping also helped the officers though, as they were doing something to help.

"There is the support and camaraderie of colleagues ... but we're just making sure we get that wraparound support for them."

Others involved following the tragedy, such as 111 operators who took calls during the massacre and officers tasked with telling families loved ones had died, would also need to be supported.

Police welfare and the association's welfare team would also work to help other emergency services, and the public, if needed, Cahill said.

Planning meetings were already under way, but sometimes the first tasks were as simple as providing food and drink to the officers still working, he said.

"And just to let them know we're here."