The entire plaster ceiling would also be strengthened.
"And because we have to put holes in the roof to put bracing in place we're going to replace the roofing at the same time."
But Mr Grobecker said the heritage features inside the building would not change.
"Things like those supporting columns. They'll be steel not cast iron because cast iron can snap in a quake."
Seats will be replaced at the same time and access for the building will also change slightly.
"It's a balancing act to retain the same number of seats to keep patronage up but also meeting the requirements of the fire regulations," Mr Grobecker said.
The contract would lift the whole building up to 33 per cent of the national building standards. At the moment it's less than 30 per cent.
"But anything in the ground will be put in at 67 per cent of the standard so if someone comes back 20 years later and says we have to lift it to 67 what we don't want to do is undo ground work we've already done."
The structural work is estimated at $500,000 and adding new seating and roof will push that sum out to about $800,000.
Mr Grobecker said the council was getting expressions of interest from firms capable of doing the job. And as a heritage building council hoped to get a contractor who had an affinity with the building's style.
"And it's a very short timeframe between June and end of August next year so we need a contractor on board who can commit resources to that. The timing is dictated by bookings for the Opera House," he said.
Councillor Martin Visser said it was job that had to be done.
"It's one of the city's most iconic buildings and this is the first of a few of them that have to be sorted out."
The others included the Alexander Library, the museum and the Sarjeant Gallery.
He said strengthening technologies were improving all the time and costs reducing.
"With the approach we're taking there's room for the contractors to bring their expertise to that design."
Mr Visser said what happens at the Opera House could mean savings in future work needed on council's other heritage buildings.