A 66-year-old man has been arrested after “brazen” twin attacks on legal offices in a regional NSW city sparked explosions and mass evacuations.

Dozens of workers were evacuated from a building on Donnison Street, Gosford, just after 2.30pm on Thursday, amid reports of black smoke billowing from the third floor.

Just minutes later, there were reports of explosions and a fire at another building further up the same street.

Fire and Rescue NSW sent more than 40 firefighters in 11 trucks to the scene.

On Friday, police issued a description and a plea for the man to come forward. Superintendent Darryl Jobson said he appeared to have set domestic LPG cylinders alight in both legal offices.

There was minimal damage at the first office, the home of the town’s Legal Aid. The damage at the second law office was more severe, and a 54-year-old worker there suffered smoke inhalation. There were no other injuries.

Michael Mantaj, director of Conditsis Lawyers, who was in the building at the time, described the event as alarming.

“I didn’t see the guy, but someone threw an incendiary device,” he told the Coast Community News.

“All staff are accounted for; fortunately, no one was injured.”

The office block was locked down so it could be ventilated before investigations could get under way.

Legal Aid NSW’s Gosford office has been temporarily shut & all staff evacuated following an explosion that caused a small fire in reception. @FRNSW extinguished the fire. None of our staff or clients have been injured. Enquiries are continuing and police are investigating.

— Legal Aid NSW (@LegalAidNSW) December 5, 2024

On Friday, police released a CCTV image of a man they sought. Later on Friday, they said they had someone in custody.

“Following extensive inquiries, police arrested a man at a home in Wallarah about 1.30pm today,” NSW Police said.

Earlier, police said they wanted to know where the suspect had picked up what was described as a “canvas bag on wheels” that he had at the second scene. He was not seen with it at the Legal Aid office.

“It would appear as though he has got another bag from a location we’re not aware of,” Jobson said.

“That bag has gone with him into a building on the corner of Donnison Street and Henry Parry Drive. He’s got up to level three in an elevator, which is an area occupied by a private law firm.

Similarly to the other attack, [he has then] ignited a similar device, being a small domestic LPG cylinder, which has caused a fire.”

Jobson said it was fortunate no one was more seriously injured or even killed.

“This is a brazen attack … violent behaviour that we will not tolerate,” he said.

“This is an incident where there could have been an entirely different outcome, taking into consideration the circumstances, and thankfully no one was seriously injured and there were no deaths on these occasions.”

The motive for the attack remains unclear.

Police have urged anyone with information to contact them or Crime Stoppers on 1800 333 000.