FENZ Deputy National Commander Brendan Nally said the strike action meant career crews would not respond to medical emergencies as usual for the hour in question.
He said the union's decision to continue to strike after a pay offer increase was "extremely disappointing".
The new pay offer would see base salaries for all firefighters increase between 8 and 19 per cent over the next two years, according to FENZ.
NZPFU Northland president Paul Ballantine said the stoppages go against a firefighter's DNA but they believe there is no other choice.
"It's an action that we don't want to take, but we feel that FENZ is forcing our hand and don't appear to be budging much on the offer."
Ballantine felt the strike was about much more than pay offers.
"It's not the money, it's welfare," said Ballantine.
NZPFU want a commitment from FENZ to provide psychological care in response to trauma, as well as an acknowledgement that firefighters are at an increased risk of cancer.
The World Health Organisation agency declared firefighting to be carcinogenic last month after research found firefighters were four times more likely to have been diagnosed with cancer than the general population.
Striking was an unprecedented move for the workforce, emphasised Ballantine.
"It really goes against our DNA to walk off the job," he said. "I've been in the fire service for 26 years and I've never walked off the job before."
"It might be hard for the public to understand, we're very reluctant to take the action but we feel we need to."
Negotiations have stretched across 13 months - including 29 days of bargaining and three days of mediation, according to FENZ.
FENZ confirmed volunteer fire brigades will continue to attend emergencies as per usual, the United Fire Brigades' Association of NZ (UFBA) said on its Facebook page.
"FENZ has also assured us that volunteers will not be requested to stand by on strategic stations where they would not normally do so," said UFBA.
