It was the morning after the night before when fire engulfed two of their buildings in Goulburn.

At about 8pm a blaze broke out in the Centrelink building on Auburn St, gutting the interior and the adjoining St Vincent de Paul charity shop.

Shortly after authorities were alerted to another outbreak in the former St Patrick's Primary School site. Mr Rampton and Mr Jones are refurbishing the building for a motel and residential accommodation.

Police said insulation batts and timber had been pushed into a corner of the building's lower level and set alight. It destroyed a small area and caused smoke damage.

Learner driver Hannah Cudaj noticed an orange glow coming from the building while driving along Cartwright Place with her mother, Belinda.

While her mother initially dismissed it as a reflection of the fire opposite, Hannah persisted. They reported the outbreak to authorities.

"I'm disappointed," Mr Rampton said yesterday of the overall devastation. "We've worked on St Pat's fulltime for the past 12 months, with a new roof, wrought iron etc. You've no idea how many people have pulled me up in the street and said what a beautiful job it is."

He said it "defied belief" to think the fires could have been deliberately lit.

The partners were yet to find out how much was salvagable in both buildings but said they were insured.

Cleaners were in the Centrelink building on Monday night when fire broke out in the second level. They along with members of the public, alerted emergency services.

It started in the section housing St Vincent de Paul Society's charity store and spread quickly through the roof and over to the Centrelink and Medicare office, Fire and Rescue NSW incident commander Bruce Fitzpatrick said.

Clothing, furniture and other items fuelled the blaze and proved especially challenging for firefighters.

Some 100 personnel tackled the blaze, including crews from Mittagong, Bowral and Harden and an army of RFS units from around the region. Later, aerial appliances from the ACT and St Andrews, near Campbelltown joined the effort.

"We had difficulty gaining access (to the main part of the fire) due to the nature of the building and the location of the shops around it, the high fuel load and aggressive nature of the fire before it was called in," Mr Fitzpatrick said.

Authorities didn't want to endanger crews and instead battled the blaze from the side.

Mr Fitzpatrick said crews worked hard to contain the outbreak to the building and said "it could have been a lot worse".

Police took little time to declare both fires suspicious.

"We're investigating but obviously two fires 150 to 200 metres apart throws suspicion on them," Inspector Rob Post said on Monday night.

Auburn St and Cartwight Place behind were blocked to traffic and Essential Energy cut electricity to the area.

Residents in upstairs units were also evacuated. An exclusion zone continued well into yesterday.

In the street, locals gathered to watch yet another Goulburn building go up in flames. Social media was abuzz with reports and photos.

But for Centrelink workers, residents and nearby business owners it was a nervous time.

Cr Carol James, who owns Century 21 real estate was among them.

"It was a shock that it came so close and it was scary for the tenants upstairs," she said.

"It's soul destroying to think that anyone would want to do this deliberately and risk lives and people's livelihoods."

Crews had their work cut out trying to douse flames on the ground but also keep the roof cooler to prevent building collapse.

The arrival of aerial appliances bolstered their efforts, allowing them to fight from the air and give clear direction to personnel below, Mr Fitzpatrick said.

The blaze was largely extinguished by 2.30am but units stayed on scene to extinguish hotspots, which continued to burn yesterday morning.

NSW Fire and Rescue area commander Ken Murphy said four crew comprising 16 personnel remained on site.

An engineer also assessed the building's integrity later in the day and the northern wall on the Vinnies' side was demolished to allow crews to extinguish the seat of the fire safely.

Forensic services investigators arrived on scene on Monday and continued their work yesterday at both fire locations. Fire investigators also arrived from Greenacre.

Mr Murphy praised firefighters' efforts.

"It was a fantastic effort considering the extent of the fire and to contain it to it's source was tremendous work," he said.

Now business owners are counting the cost of Goulburn's second major CBD fire this year. In January the Greengrocer cafe was also destroyed.