The 1887 classical revival Auburn Street building appeared small as architect Ian Brewster surveyed it from the outside.

Inside, he found a network of small rooms large and small branching off from a long central corridor.

"It was a much larger building and that's what made it so charming. It's a bit like a dollhouse," the Brewster Hjorth director and CEO said.

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Three years ago, Mr Brewster and his team were appointed to design Goulburn's new performing arts centre in and around the structure.

They were among guests at the Friday night's official opening of the $18.95 million facility, following two years' construction.

Mr Brewster said when the council first approached him about the design, it was more interested in conserving the building's heritage. He stressed that it also had to work effectively as a theatre, otherwise there would be community backlash.

Input from the soon formed performing arts working party became vital.

The constrained site posed significant challenge for the architects, and later, Zauner Constructions.

"It is probably about five metres too short on in each direction but we really wanted to make it work," Mr Brewster said.

"I think it does work and makes the whole experience more intense. We didn't skimp in any part of the auditorium or stage."

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Blending the old with the new also proved challenging. A 1930s addition by architect HC Manfred, the son of the original building's designer, was demolished for the new extension. This section incorporates the atrium and 400-seat auditorium, at the same time tipping its lid to heritage. The outline of the original building, for instance, is etched into the northern exterior wall.

Other pleasant surprises surfaced. Builders, Zauner Constructions, discovered the original ceiling in the town hall chambers one day after "poking their head up through a manhole."

"Everything was preserved and untouched so we exposed the timber and used its diagonal pattern throughout the building," Mr Brewster said.

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The architect was also mindful that the fence line of Goulburn's first jail went through the building's centre. Bushranger Thomas Whitton was reportedly hanged in the vicinity in 1840. Archaeologist Wendy Thorpe did not find any remains and it was thought Whitton was buried near the front of the building, where excavation wasn't necessary.

A section near the cafe has been cheekily named 'Trapdoor' as a result.

A vault where the council kept records and cash has been converted into the box office.

Whereas EC Manfred incorporated arches into his original design, his son had a penchant for squares. Brewster Hjorth restored many of the arches.

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Mr Brewster and his team were thrilled with the final product, which won glowing praise on Friday night.

"The result speaks for itself," he said.

"I think the GPAC will make a stir in the region and people will seek it out."

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