The 21 Kiwi firefighters crossing the Tasman to join the battle against massive bushfires in Australia are heading into a "highly dynamic environment", says the man who became known as the "face of the Port Hills fires".

Richard McNamara, an incident controller during the 2017 Port Hills fires in Christchurch, has fought major wildfires in the US and Australia as well as New Zealand.

Now the aviation lead at Fire and Emergency NZ, McNamara said some of the fires he had been involved in fighting had been on the same scale as those now burning in Australia, but not of the same level of consequence.

The catastrophic fire conditions in major cities, with the potential to severely impact large populations, was unprecedented, McNamara said.

Conditions were at the worst end of the spectrum given the heat coming out of the Australia interior, the high winds and drought conditions.

"It sucks the breath out of you. It's like trying to work in an oven," McNamara said.

"What happens in a large wildfire. It's driven by the terrain it's burning in, the fuels and the weather that's driving it. And fires can get so big they develop their own weather system," he said.

"It's a highly dynamic environment you are putting people into. Firefighting is about trying to predict that very dynamic fire environment, so that we safeguard our people and the public, and also can do good.

"Sometimes you have a good day and do all you possibly can do and still get beaten by the fire," McNamara said.

"It's a task of managing risks while attempting to do the most good, and it's a very dynamic environment to work in."

The Kiwi contingent was made up of exceptionally good firefighters, many of whom would have had experience of large wildfires.

"They will do the utmost they can for the people most at risk - and we bring our people home."

The Government announced the deployment of the 21 firefighters to Queensland on Tuesday. The contingent is made up of six three-person crews, two taskforce leaders and a liaison officer. They are due to begin operations on Wednesday, and are in addition to six New Zealand personnel already in New South Wales.

Internal Affairs Minister Tracey Martin said the numbers deployed were likely to change at short notice.

"The request to provide this assistance is recognition of the high regard in which our Fire and Emergency personnel are held internationally," Martin said.

"Deployments such as this also provide valuable development opportunities for our personnel. Some of the lessons learnt on past overseas deployments were put into practice at the Tasman fires in February."