"You're dealing with science, physics - we learn a lot about how to monitor the behaviour of smoke," said Mr Herrara, who is based at Drummoyne in Sydney's inner west.

But what Mr Herrara and his firefighting colleagues encountered in the CBD on August 13 - a blood-spattered man walking down York Street with a knife - could hardly be foreseen.

Mr Herrara and his crew - station officer Mike Stuart and firefighters Mitchell Bennetts and Bennett Gardiner - found themselves in the midst of what police would later describe as "terrifying carnage".

Mert Ney, 20, has been charged with murdering Michaela Dunn, 24, in a Clarence Street unit and stabbing Linda Bo, 41, in the shoulder at the Hotel CBD before wandering through the city.

Mr Bennetts told reporters on Monday he saw people running for their lives down York Street as he was driving the firetruck carrying his three colleagues.

"I saw the man, he had a mask, covered in blood, with a big knife," he said.

"I started air-horning the truck. If anyone's ever heard it in the city, it usually scares the hell out of all the pedestrians.

"I put the truck in park and ran to the back. At this stage, he was up on a car, looked at me ... and we made eye contact."

Mr Bennetts and Mr Herrara then kicked into gear - they ran after the man while trying to prevent him from assaulting others.

With the help of several bystanders wielding chairs and milk crates, the alleged offender was felled and duly apprehended.

"It was such a short time in which we had to react. We just wanted to help people, that's it," Mr Herrara said.

"This was a person, so it was an unpredictable situation. We've never faced this type of emergency before, I haven't, but I still tried to apply my skills in managing the incident and control it as much as possible."

NSW Premier Gladys Berejiklian on Monday lauded the firefighters for their efforts.

"We know from reports that when the perpetrator saw the men in uniform, It caused him to think twice about continuing his actions," Ms Berejiklian told reporters.

Australian Associated Press