Otto said Whangārei firefighters were proud that they had not yet operated short-crewed this year or last year, but staff had been putting in many extra hours to prevent this.
"That's meant a huge sacrifice for their families and their personal lives by putting in those extra hours."
Recruitment was made more difficult by the low wages offered to recruits, who earned just over $22 an hour, he added.
"Trades and other career options out there pay much more competitively and don't put such a burden on our health, your psychological wellbeing."
The union said in a statement there was a "fire crisis" in New Zealand, with some firefighters working up to 100 hours a week as Fire and Emergency NZ (FENZ) has not employed enough career firefighters.
NZPFU also wanted a commitment from FENZ to provide psychological care included in a new collective agreement, as well as an acknowledgement that firefighters are at an increased risk of cancer.
"We know that firefighting exposes you to psychological trauma and to carcinogens," Otto said.
"By working extra overtime, and above your contracted hours, you increase your exposure to carcinogens in the post-fire environment. That all has a cumulative effect on your career."
Another issue was the impact of responding to medical responses, which firefighters have been doing since 2013.
"Firefighters have never been remunerated for this work, which takes a significant toll on their mental health and wellbeing," the union said in their statement.
More than 99 per cent of NZPFU members voted to reject the offer from FENZ, which gave two-thirds of staff a 1.5 per cent pay increase over two years.
Firefighters planned to strike indefinitely until a collective agreement was reached.
FENZ chief executive Kerry Gregory said in a statement that the organisation had been negotiating with firefighters for some time, but would not respond to their claims outside the bargaining process.
"I'm disappointed the union has rejected our offer and decided to take this industrial action. But I acknowledge their right to do so.
"My focus is on keeping our communities and firefighters safe and working constructively through this current situation."