An overdue alpine fault earthquake will strike "out of the blue" and cause widespread death, shut down power generators, create tsunami within New Zealand and overwhelm emergency services, experts warn.
The major quake will cause intense shaking and rupturing along hundreds of kilometres of the fault line bisecting the South Island, geology experts Tim Davies and Mauri McSaveney predict.
"The most likely time [for the quake] is now. The next most likely time for it to happen is tomorrow," Associate Professor Davies, of Canterbury University, told the Natural Hazards Management Conference in Christchurch.
"The longer the delay, the bigger it will be. It will occur with no recognisable warning. We can't manage it - we have to adapt to it."
The pair have outlined a nightmarish scenario in the aftermath of the quake and are urging people to be prepared as best they can. Overseas help would be needed when the quake struck.
"There will be death and injuries, especially in the [Southern] Alps and West Coast. Rescue services and medical services will be overwhelmed, and remain so for weeks in places."
"Shaking damage and land instability will disrupt surface transport for months, tourists will be trapped, and distribution of vital supplies (eg, food, fuel) will be limited.
Hydro stations will shut down immediately and may be slow to restart, power reticulation will be damaged. Only satellite phones will remain in use."
"No services will be as normal."
The intense shaking would cause landslides of millions of cubic metres, damming rivers and later causing floods. Aftershocks could continue for months.
"Landslides into lakes and fiords may cause tsunami, as may the collapse of river deltas in lakes or the sea. Queenstown, Milford and Wanaka are likely sites of tsunami damage."
The pair say the Alpine fault quake is a certainty. It is only a matter of when.
"The interval since the last event (in 1717) is longer than any interval between known earlier events."
Plans needed to be developed now for the worst-case scenarios, with relocation of "obviously perilous" facilities, buildings and infrastructure, with spare equipment and supplies stored in safe locations.
Tour bus operators were urged to stock up on food and supplies for their customers who could likely be trapped for days in isolated locations.
Experts at the conference have also warned of the risk of a deadly ocean tsunami striking New Zealand.
The country had experienced tsunamis in the past, but modern shoreline development has made it much more vulnerable, according to GNS expert Kelvin Berryman.
If warnings and evacuations were not carried out in time, an estimated 5000 people would be killed by waves up to 8m high on the east coasts of New Zealand.