If a big earthquake hits Wellington, the city could be cut off for four months, power and gas not fully restored for three months and without sewerage for several months, a new report warns.

The Wellington Lifelines Group, made up of utility operators and civil defence authorities from local and central government, released a report today examining the restoration of key infrastructure after a major rupture of the Wellington Fault.

The report comes after six months of research by the members, the group's chairwoman Fran Wilde said.

"This report has taken a worst-case scenario - a Wellington Fault rupture measuring 7.5 on the Richter scale,'' Ms Wilde said.


"It is important to stress that the chances of a quake of this magnitude in the near future are small. A major rupture is predicted to occur approximately every 840 years, and since the last one was 300 years ago, hopefully we have some time to build our resilience.''

If a quake measuring 7.5 in magnitude did strike the capital, ground alongside the fault could shift sideways by 5m and lift one side by 1m.

Ms Wilde said strengthening infrastructure was a costly job that could take many years to complete.

But residents should put the danger risk into perspective, as the report sketched a scenario that had only a one-in-10 chance of happening over the next 100 years, she said.

"Nonetheless, people should not be complacent."

Living on a faultline meant no amount of strengthening work could remove all risk, she said.

Jenny Rowan, chairwoman of the Wellington Region Civil Defence Emergency Management joint committee, said residents needed to understand the report's implications for them and their families.

"People and organisations need to be aware of the catastrophic disruption of a major earthquake and ensure that their planning takes it into account. A few bottles of water scattered round the house are no longer enough."


The region's electricity provider, Wellington Electricity (WE), warned residents needed to be prepared to help with the cost of seismic strengthening.

WE's chief executive Greg Skelton said the company would seek consumer feedback next year on what level of strengthening of the local electricity network they would be willing to support.

"The question is whether people want to pay more now to ensure the power comes back on more quickly after a big earthquake."

The aftermath of a 7.5-magnitude quake include:

* Wellington would be cut off for four months or more by damage to the main routes and rail lines;.

* Most of the region would be without gas for nearly three months and without power and water for at least three weeks.

* Telecommunications services would be out for 10 days.

* Restoring sewerage would take several months.

* Food, fuel and materials to a cut-off Hutt Valley might have to be barged to Seaview Marina or Petone beach.