Investigators released a CCTV image of a man in his 60s they wanted to speak to over explosions and fires at Gosford on the NSW Central Coast on Thursday.
A 66-year-old man was on Friday arrested at a home in Wallarah, about 30km north of Gosford.
"We believe that this is a targeted attack and not a random incident," Superintendent Darryl Jobson told reporters earlier in the day.
"It's obvious that there was a reason why the man went to both locations, and we're keen to ascertain what that reason is, but at this stage, we don't know."
Emergency services were called to buildings on Donnison Street, Gosford about 2.20pm following reports of explosions at a complex that contained offices for Legal Aid NSW and other state agencies.
A Legal Aid spokeswoman said staff were evacuated and the office temporarily shut following an explosion that caused a small fire in the reception area.
"No members of our staff or clients have been injured," she said.
The man went to the offices of a private legal firm after the initial incident, police said.
"Similarly to the other attack (the man) has ignited a similar device, being a small domestic LPG cylinder ... that fire was more substantial," Supt Jobson said.
A 54-year-old man was treated for smoke inhalation but did not require further medical aid.
"This is an incident where there could have been an entirely different outcome ... thankfully, no one was seriously injured," Supt Dobson added.
The arrested man was taken to Wyong police station on Friday but had not been charged.
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