Hydro dam operators say they are well prepared for any rupture in the Alpine Fault.
Canterbury's revised civil defence emergency management plan says there is a 65 per cent chance the fault could cause a magnitude 8 quake in the next 50 years.
GNS natural hazards research manager Dr Kelvin Berryman said "65 per cent was on the high side".
"But you can see where it comes from, it's from combining all the likelihoods of the rupturing occurring somewhere along the fault," he said.
The Alpine Fault runs hundreds of kilometres along the western side of the Southern Alps from Marlborough to Milford Sound, where it heads offshore, and marks the boundary of the Pacific and Australian plates.
The two main operators of the Upper Waitaki hydro system said they were well aware of the risk.
Genesis Energy spokesman Richard Gordon said its recent $125 million repair of the Tekapo canals would have sufficiently strengthened its part of the system.
"The recently completed upgrade to the Tekapo canal and canal bridges not only improved the performance of the canal but also improved its seismic resilience to movements within distant faults," he said.
Meridian Energy, which owns and operates six of the hydro stations from lakes Pukaki to Waitaki, said it was similarly prepared.
"The work that Meridian has completed confirms that all the dam structures in the Waitaki Valley and at Manapouri will perform well in a seismic event and will withstand a magnitude 8 earthquake," spokeswoman Michelle Brooker said.
Meridian is in the process of carrying out a $67m repair and upgrade of the Benmore power station.
The work began in 2008.
The Waitaki hydro station - the oldest in the system - will undergo a $40m overhaul over the next four years.
Berryman said if the fault ruptured, the likelihood of the dams collapsing would not be great.
"It's likely to affect the South Island's energy supply for some time . . . but it's really the roading network that would be mostly hammered," Berryman said.
"Really, the focus should be on planning and disaster readiness, and strengthening our building code.
"We just don't know how long it would take to repair the roading network."
Berryman said the Alpine Fault was late in its cycle.
"It's nearly 300 years since it last ruptured. If it ruptured in the next 12 months, it wouldn't be a huge surprise.
"Equally, a rupture could be up to 90 years away.
"The bottom line is that if it doesn't rupture in our lifetime, then there's a high chance it will rupture in the lifetime of the next generation," he said.
- The Timaru Herald