A big quake could leave Wellington reliant on barges and helicopters for survival, new disaster predictions show.
Massive landslides would cut off Porirua, the Hutt Valley and Wellington from the rest of New Zealand and from one another, with inbound roads taking up to four months to clear.
All three areas would rely on barges and helicopters to ferry in food, clean water and vital medical supplies.
The would be no power, water or gas for at least three weeks, and for more than two months in some Wellington suburbs.
Wellington city would be cut off for the longest, with no power for at least two months and no gas for three.
It would take at least 55 days to open State Highway 2 between the Hutt Valley and Wellington, and three weeks to connect the capital and Porirua.
The grim predictions are contained in a report by the Wellington Lifelines Group, which includes 20 of the region's major infrastructure companies and civil defence authorities.
It is based on a "worst-case scenario" of a 7.5 or higher magnitude quake, which has a less than 10 per cent chance of striking within the next century.
The report also reveals just how vulnerable the region's basic services would be in a big quake.
Its hills funnel vital roads, powerlines and water mains into bottlenecks, often crossing Wellington's major fault line, meaning it would take considerably longer to restore services in Wellington than it did in Christchurch after the February earthquake last year.
It would also be harder to keep the city supplied, with the report noting Wellington's supermarkets have no major backup storage in the region.
Regional Emergency Management Group co-ordinator Bruce Pepperell said road access would be the biggest priority. "Parts of the region will be completely cut off from others."
The stretch of State Highway 1 sandwiched between the coast and cliffs along Centennial Highway would probably be the most difficult to clear.
"If it is necessary to blow the top of the hill into the ocean [to open a road], that is what will happen."
Greater Wellington regional council chairwoman Fran Wilde, who also chairs the lifelines group, said the report was part of a wider resilience review by infrastructure firms after the Canterbury quakes.
However, many improvements would take years, and the public would need to decide on the tradeoffs between cost and quake resilience, she said. "Unless you have a limitless budget, there is no quick fix."
Wellington Electricity Lines has already said it will need to charge more for electricity to strengthen its infrastructure.
The lines company has 300 substations in potentially quake-prone buildings, and the mostly underground power network is vulnerable to quake damage.
Chief executive Greg Skelton said the company would seek "customer feedback" on a price hike, which would require Commerce Commission approval.
The regional council is also considering new emergency water reservoirs, and briefly entertained building a desalination plant on the south coast.
Both Pepperell and Wilde stressed the latest predictions were for the worst possible event, and a smaller less-damaging quake was far more likely.
"It is not a cause for everyone to go pick up their bags and leave," Wilde said.
The report says that, whatever strengthening is built into services, it is equally important that people prepare personally for a big disaster.
- The Dominion Post