A large earthquake offshore in the Puysegur Trench could send a tsunami surging towards the Southland coast before official warnings could be issued.

That's the finding from research that Emergency Management Southland commissioned from GNS Science, and is the basis for a new awareness campaign that the region's civil defence staff are about to launch in small communities around the southern coast.

People who live on or near the sea are being urged to take the initiative if they feel a large earthquake - one so powerful that they have difficulty standing, and lasting for 30 seconds or more - and immediately move inland or up a hill.

Emergency Management Southland's Manager, Neil Cruickshank, said that the GNS report had reassessed the risk of a damaging tsunami from an earthquake close off shore reaching Riverton and the New River Estuary, and it was higher than previously thought.

"The reality is that if there is a magnitude 8.7 earthquake in the Puysegur Trench, we would not have time to issue formal warnings and evacuate people from the coast. People will have to use their initiative and move uphill or inland immediately."

The worst case scenario from the GNS Science model shows that a tsunami surge of up to 7 metres above normal sea level would hit the coast along Te Waewae Bay, the Riverton coast, Oreti Beach and the west coast of Stewart Island. Riverton township would also suffer major flooding up to 7 metres all through the residential areas of Taramea Bay, Mitchells Bay, the flats on the northern side of the township and the Jacobs River estuary. Tide speeds would increase from the normal 1-2 knots to around 15 knots.

Flooding is also expected in the New River Estuary, with increased tides pushing up the lower reaches of the Waihopai River. While Otatara would not be seriously affected, the Stead St stopbank would likely be overtopped and parts of the Bond St industrial area and airport would probably be flooded.

Water levels within Bluff Harbour could rise up to 3.5 metres, with 15 knot tidal flows - around four times normal - and outside the harbour entrance, surges of up to 6 metres could be expected.

The south east coast around to Waikawa could experience tsunami waves of up to 5 metres.

Mr Cruickshank said that a similar level of wave surges and damage could be expected from a tsunami generated by a 9.4 magnitude earthquake off the coast of Peru, but there would be at least 12 hours warning so Civil Defence and Police would have time to evacuate vulnerable areas.

Emergency Management Southland is likely to commission more research to assess the risk to other coastal townships in Southland, including Fiordland. In the meantime, Mr Cruickshank said people in Milford Sound or anywhere around the coast should take the same sensible approach if they felt a powerful earthquake.

"The reality is that you won't know whether the earthquake that you feel is located offshore, on shore, close by or on the Alpine Fault. Don't wait to be told - act now. Move inland or uphill, listen to the radio and wait for instructions."

Places of safety were still being identified, but people living on the east side of the bridge in Riverton should move to the Racecourse, while those in the Riverton Rocks and Taramea Bay area should go up the hill.

At Curio Bay, people are advised to go inland past Niagara, while in Bluff they should move up Bluff Hill.

Emergency Management Southland will be providing more specific information to people living in the most vulnerable communities.

In the meantime, the tsunami report is available on the organisation's website www.civildefence.co.nz and anyone with questions can contact EMS staff on 03 211 5115