Because of its geographical and geological setting, New Zealand and its territories are particularly vulnerable to natural hazards. There are many reminders that we live in a high-risk zone.

Mt Ruapehu has erupted nine times in the last 150 years. The great Napier earthquake struck less than 80 years ago. Rangitoto volcano, just 7km from Auckland's CBD and barely 2km from the North Shore's CBD, last went off around 1405.

The most powerful earthquake in New Zealand's recorded history struck Wellington only 155 years ago. Still that city continues to expand along a major geological fault zone. A tsunami from Chile produced a 5.5m tsunami in Lyttelton Harbour on May 22-23, 1960 and affected other east coast harbours.

It is widely held that tsunamis are rare, and popular wisdom holds earthquakes responsible for the killer waves. Neither of these beliefs is entirely correct.

A tsunami is a surge of water or a series of surges generated by an impulsive, shock-displacement of ocean water that can occur anywhere.

Like earthquakes, volcanoes can also cause these surges, and often do. One of the most destructive tsunamis in recent history occurred when the island volcano of Krakatoa erupted in 1883.

Submarine landslides, which can involve thousands of cubic kilometres of material, also have the power alone to generate a tsunami.

Tsunamis can also have their origins in space. Australian geographer, Ted Bryant, reminds us of the role of meteorite impact with the ocean, when, on February 22, 1491, a meteorite strike caused tsunami waves in excess of 130m high along the Australian coast.

Mention disaster management and most people inevitably think of physical events such as these. But the natural events themselves do not necessarily present the greatest risk. The greatest risks come from situations that are not beyond our control.

A civil emergency is to a large extent a function of the adequacy of preparedness planning. How soon could Auckland be evacuated?

There is a single motorway out of the isthmus that is the Auckland urban area. One major harbour crossing means there are no alternative routes. Main feeder roads head for this point and easily become congested.

Approach lanes to the harbour bridge are close to the high-tide mark and highly susceptible to flooding from storm surges.

A report on the vulnerability of the Auckland Harbour Bridge showed even a modest earth tremor could fracture supports for the four clip-on lanes. The result would mean traffic chaos, the consequences of which would persist for months. Ferries can provide little relief.

The economic effects would be immense. It is only after calamities occur that we recognise how vulnerable we are.

Too often emergency management is seen to be concerned with little more than saving lives. In fact, more frequently it entails reducing the risks to people, buildings, infrastructure and economic activities.

The focus is on risk reduction, readiness, response and recovery. In this context, government and local authorities have the responsibility to minimise social vulnerability and have a duty to promote community resilience through enlightened planning.

Real crises provide an opportunity to assess community preparedness, but if it is not adequate the cost is high. Clearly, it is preferable that we identify perspectives on emergency planning that prevail among many local authority managers and politicians before a real crisis occurs.

Emergency and disaster planning involves much more than saving human lives. It entails protecting the integrity of infrastructure, such as communication networks, electricity supply and roading. Failure of this infrastructure can be catastrophic to the community in terms of social, economic and personal hardship and loss. A critical ingredient is the need to plan for contingencies or mitigation before the event.

The crisis potential for Auckland is high for a variety of reasons. The main one is that Auckland is situated on relatively narrow strips of land surrounded by water, with a limited number of land-based access routes and numerous potential bottlenecks. North-south transport links are vital, increasingly so as the urban area develops along a predominantly north-south axis straddling the eastern edge of the Waitemata Harbour.

How long would it take to restore Auckland's Harbour Bridge and at what cost? The interruption to the life of the country's largest city would be immense.

Auckland City Civil Defence Emergency Management Local Plan 2005 conservatively lists the chance of infrastructure failure that is unrelated to natural hazards as "likely to exceed" a 10 per cent probability of occurrence in a 50-year period. This Auckland local plan coolly states economic consequences are "inevitable" and that "the flow-on effect on tourism and trade likely to be long term because of the resultant loss of credibility."

For earthquakes the plan states that although the Auckland isthmus is one of the least seismically active areas in New Zealand, the population concentration and the potential hazardous substances inherent in the diverse industry clusters, means the danger from a local or near-field event cannot be ignored.

A serious event is given a 43 per cent probability of occurrence in a 50-year period and an extreme event a 7 per cent probability. The latter would cause "significant damage ... loss of life and high social disruption ... Loss of key lifelines: communications - up to 7 days; water, sewerage and stormwater - up to 6 months; road/rail - up to 6 months; energy - variable, but could be several months [with] severe economic losses and a recovery timescale of up to 20 years."

According to the local plan there is a 49 per cent probability that a tsunami will hit Auckland in a 50-year period. The biggest question in natural hazards research is not if a severe natural event will happen, but when.

* Chris de Freitas is an associate professor in the School of Environment at the University of Auckland.