Mr Ryall said Canterbury had already faced capacity problems due to the ageing and growing population, but the Christchurch earthquakes had made the situation a lot worse.
"The price tag for the redevelopment could be in the vicinity of $600 million," he said.
"We need to meet the future needs of the people of Canterbury and make sure it is affordable."
One option being considered was a "modified greenfield", which involved rebuilding most of the clinical wards at Christchurch Hospital, including a new acute services wing. It would also include a new specialist centre for older peoples' services at Burwood Hospital.
However the Association of Salaried Medical Specialists executive director Ian Powell objected to the proposal, saying it was code for privatisation.
"Their driver will be profits, pure and simple. There are big bucks to be made here. It would be like asking a panel beater to design a traffic intersection."
Public hospitals are highly integrated organisations designed to handle the most complex and urgent pressures in dealing with patient lives, he said.
"Each part of the hospital depends on the other parts to succeed. The last thing they need is the introduction of fragmentation that this sort of privatisation would create."
Labour's health spokeswoman Maryan Street said Mr Ryall had dumped the news on the eve of Christmas to try to hide it.
"Make no mistake - this is the beginning of the privatisation of health services. It is wrong because it will undermine the public health system, it does not make economic sense, and this model has failed internationally."
She said the Government could borrow the money to build the hospital more cheaply than the on-going costs of paying the private sector to build and own the facilities.