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Chief executive Chris Fleming said the spending was essential.
"Every dollar we invest in the existing facility is a dollar that’s going to be regretted in the long term, but we can’t be ostriches.
"We have to keep the hospital running.
"We simply won’t be able to make do with what we’ve got now for the next decade," he said.
Despite the work being urgent, and upgrades having a limited shelf life because of the new hospital, it would be months before it could be approved.
A large portion of the money would be used maintaining the hospital’s "failing infrastructure" such as worn-out electrical, steam and heating systems.
An upgrade of the emergency department was a priority and an extra operating theatre might be built, but the detail had not been worked out yet. Most of the money would be spent in the hospital, but some would be spent providing services off site. The programme would involve shuffling some services around the hospital and "repurposing" certain areas.
"We’re looking at the whole hospital and saying, ‘Is there something we can take out so we can repurpose the space’?"
Earlier this month, ED clinical leader John Chambers called for an urgent upgrade and expansion, as the department was overflowing with patients on busy days.
Mercy Hospital’s capacity would be considered before a decision was made about building new theatre space.
The hospital was not keeping up with surgery demands but Mr Fleming wanted to avoid straight outsourcing where possible. He preferred to find other places for some services but relocate DHB staff to provide them.
Mr Fleming had hoped to complete a business case application before Christmas, but it had been delayed because he wanted to consult clinicians. He hoped to put in the application about March and gain approval around the middle of next year.
It was possible the Government would tell the DHB to fund all or part of the work itself from existing budgets, he acknowledged.
The amount of money sought had not been finalised but would be close to $100 million.
Mr Fleming is working on a seven to 10-year timeframe for the new hospital.
The Government has promised to speed up the build, but has not laid out a firm timeframe.