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Brown said the use of base isolation units - the same as at Wellington's Beehive - were being investigated but nothing had been decided.
They showed off a sales suite on level one of the building beside the old theatre, telling how 290 to 330 apartments were planned in a 39-level tower.
"The budget for the St James project is still being formulated as the evaluation process continues. It won't be known for some time," said a written statement released today.
"All required seismic and safety work will be done with the priority of re-opening.
"Upon opening, a staged improvement programme will occur over a number of years.
"As a Category One listed heritage landmark, all work on the St James will be scrutinised by Heritage New Zealand.
"Long-term ownership structure of the St James to be finalised in due course," the statement from Relianz said.
Gibbon and Brown said redevelopment of the site was planned to start later this year or early 2016.
The apartments and a big new shopping centre are all planned for 302 Queen St alongside the St James and Gibbon stressed there were never any plans by this owner to build over the top of the old building, opened on July 5, 1928.
Instead, the new tower would rise alongside it, he emphasised.
Resource consent for the tower was granted in 2013, Gibbon said, and a major sales programme for the apartments starts tomorrow.
Deposits of only $1000 are being sought but Gibbon said already 170 of the units had been sold "eighty per cent to locals and 20 per cent to overseas", he said referring to the Chinese migrant owners' social media pre-launch.
Gibbon said apartments would sell from $500,000 and go up to $1 million.
An eight-level L-shaped podium will be built alongside the theatre with the tower above that.
Brown said floor plates would be 500sq m to 600sq m.
Gibbon said the St James was only about 30 per cent of new building standards.
Relianz also owns that theatre and Brown said millions of dollars were needed to bring it up to current building regulations.
"In a few weeks, we'll open up the lobby so people can look," he said, referring to the cafe plans in the foyer with ornate architectural features.
Demolition clauses have now been exercised in leases on about 10 Queen St shops ready for the big build.
Gibbon said two levels, or more than 2000sq m of retail development, would be built, including a supermarket in the basement of about 1200sq m and double-height fashion shops in the ground floor of the tower.