Rain bucketing down across NSW prompted a near hysterical reaction to new tsunami evacuation maps that were published on Friday.

Within an hour of posting its new maps, the NSW State Emergency Services (SES) updated its Facebook page to address fears and panic.

"Important: This is not a tsunami warning."

By Saturday,  the SES maps had been shared nearly 6000 times on Facebook, liked and disliked 2400 times by many readers - many of whom used Facebook emoticons for unhappy and scared - and attracted nearly 4000 comments.

The evacuation maps cover the entire NSW coast. They highlight in red the areas where residents would - in the one-in-10,000 year chance of a serious tsunami - be required to seek shelter and move to higher ground of at least 10 metres or evacuate to a safer zone.

They show those areas that lie under 10m above sea level, are 1km or less inland and are 10km up an estuary. Those people who live in the red zone, including many Sydneysiders with nice harbour views, would be asked to seek shelter.

The SES stressed that the maps didn't show areas that would be inundated in the event of a tsunami, very likely a much smaller area.

"They represent the areas considered most at risk from modelling of potential tsunami and effects observed elsewhere. The NSW SES is working on getting more data and refining maps to show potential inundation and evacuation routes in the future," it said on its Facebook page.

In the extraordinary event of a land-based tsunami which would cause damage inland, residents in large parts of Sydney near the harbour, in the inner west, around Botany Bay and beaches would be a risk of evacuation. Most of Newcastle's inner city residents would also need to evacuate.

The maps represented the worst-case scenario, said SES spokesman, Phil Campbell.

They were based on data from Geoscience Australia, which showed the worst-case scenario. While the worst-case scenario occurred in Australia about one in every 10,000 years, flooding of foreshore land occurred every several hundred years.

"In the absolute worst case, the Corso at Manly would have water five metres deep going across it, " he said. A smaller one would very likely just result in some water going a few metres in from the sea.

Within an hour or two of the SES posting its maps, the responses on Facebook suggested that some members of the public had heard the weather forecasts for continuing heavy rain, seen the maps, and put two and two together to think a tsunami was on its way.

One woman asked the SES if there was a reason it was posting the map "with a storm coming". Another person asked why it has posted the maps "right now when we are expecting a big rain event here in the next few days. I feel a LITTLE worried," wrote a woman called Narelle Lackenby.

Another woman, Carli Drewe, asked if rain caused tsunamis.

The answer is no.

NSW SES' new evacuation maps scared some people on Facebook. 

Mr Campbell said the launch on Friday had been planned for months, and it was unfortunate that the developing weather situation coincided with their release.  A range of community events had been planned to discuss the maps. 

"We are apologetic to those people who may have linked the two," he said.

The maps had been designed to grab attention, and prompt discussion and draw attention to the need for planning for this very unlikely event. "The risk of a tsunami was very low, yet the consequences are very high," he said.

According to the SES' Tsunami safe page, NSW has a documented history of tsunami. Most have been relatively small and generally less than one metre on the tide gauge records.

Around every six years, NSW feels the effect of a marine-based tsunami,

Since 2007, four tsunami events originating from earthquakes off the Solomon Islands, New Zealand, Chile and Japan, have affected Sydney.

The 1960 Chilean tsunami was the largest recorded on the NSW coast in recent history, causing about £24 million of damage.

Mr Campbell said this marine-based tsunami sunk a coal barge, water came ashore near Batemans Bay on NSW's south coast, and the effects were felt on waterways as far north as Windsor in Sydney's north-west.

"About every six years you get a marine tsunami, sometimes you don't notice them, but sometimes you get the ones that rip boats from their warnings."

A tsunami buoy would give Australians prior warnings of most tsunamis, though experts like Campbell say an undersea landslides off the continental shelf would be harder to detect.

For safety and preparedness advice go to www.tsunamisafe.com.au