That's a better outcome than most renovations, let alone a project that was like "digging an underground mine in Broken Hill" under Australia's most famous building, said Bob Leece, the chairman of the Sydney Opera House Building Committee.

After three and half years of construction that has disrupted pedestrian access and put an ugly spot on the neck of one of Sydney's shiniest icons, nearly all of the Opera House's enormous forecourt will be fully restored to the public by Christmas. It's been the largest capital works project since the Opera House opened in 1973.

By digging a two-lane road beneath the Concert Hall and the Opera House, around 1000 heavy vehicles a week will be removed from the forecourt. They will be redirected to an underground and automated loading bay that can handle four trucks. Five new lifts will carry freight and supplies. The forecourt was also made safer for pedestrians by the removal of some curbs.

Some of the excavation involved changes to historic 1857 Bennelong stormwater channel.

When the Vehicle Access and Pedestrian Safety (VAPS) project was originally announced in 2011, it was forecast to be completed by 2013. But when contracts were put out to tender, the four major contractors including John Holland which was awarded $100 million, said that deadline was unrealistic, Mr Leece said. A new deadline of September 2014 was agreed.

"This is really a large engineering achievement under our most famous building. Obviously it had to be done in a methodical way, step by step through 80 odd sequences, but some of which have taken a bit longer than we anticipated, but it was as hard as it gets," he said.

"We have built a very big hole about 12 to 15 metres under the water level, and we have tunnelled through under the Concert Hall and Opera Theatre, and we've extended and put in big large lifts," he said.

The project will be completed slightly later, but on budget, Mr Leece said.

Pile rigging machines had to be built in Germany to cope with the low-height clearance so that the hole could be anchored below water level to "stop that the big hole from floating out and popping up out of the ground."

 

In a video describing the work, Greg McTaggart, the director of building development, said he didn't think any other performing arts building in the world would attempt major building while keeping the building open.

"We didn't lose one performance in [the] entire three and half years of undertaking all these works. The place was operating entirely without disruption, which is quite extraordinary," Mr Leece said.

The construction involved the development of a 3D model, showing the interaction of the old construction and the new.