A rare weather event that has happened only twice since 1900 is almost certain to dump excessive rain on flood-ravaged communities along Australia's east coast this summer, an expert says.
The east coast has already endured two seasons of La Niña over the past two years, a weather phenomenon that causes higher than average rainfall.
La Niña events typically run during spring and summer. La Nina years, therefore, refer to the second half of a calendar year and the beginning of the following year.
The Bureau of Meteorology (BOM) has said there was a 70 per cent chance of another La Niña event, the third in a row, though other agencies, including the UK and US, have said it is already underway.
Agus Santoso, from the climate change research centre at the University of NSW, said La Niña occurs when stronger than normal trade winds blow east to west across the Pacific towards Australia.
That causes warmer water to accumulate on the ocean surface around Indonesia, which forms clouds and dumps more than the usual amount of rain on the east coast of Australia.
Dr Santoso said triple La Niñas, where the event happens three seasons in a row, has happened only twice since 1900 — from 1973 to 1976 and from 1998 to 2001.
The BOM also includes 1954 to 1957, though Dr Santoso said the final La Nina was more neutral.
In February 1954, 30 people died in the NSW northern rivers region, when Lismore, Murwillumbah and Casino flooded.
A year later, nearly all of the NSW river systems were flooded and 50 people died, with the Hunter Valley towns of Maitland and Branxton hit particularly hard.
Lismore, renowned as the most flood-prone city in Australia, has experienced 32 major floods since 1900, according to local council records.
The city was hit with major floods in two of the three La Niña years in the 1950s triple event, all three years in the 1970s triple event and once in the triple event around 2000.
The major floods in those years all happened in February or March.
Lismore was hit with its largest ever flood in February this year, when the Wilsons River peaked at 14.4 metres, smashing the 1974 record by more than two metres.
Dr Santoso said La Niña events tend to originate in winter and peak towards the end of the year.
"La Nina events, overall, will start to die away in March, April and May," he said.
Flooding is caused by months of rainfall in spring, soaking the soil and water catchments.
"And then it starts to peak and lasts for a few months until we get more rainfall, so that will contribute to flooding," he said.
Flood historian Margaret Cook, from the University of the Sunshine Coast, said that more floods so close on the heels of recent events had caused "flood fatigue".
"Psychologically, it's going to be really hard, because it's a triggering event," she said.
"And some people won't be as resilient and that's very real.
"Some people could be hit for a second or third time in the one year. It's very hard to keep getting on with it."
However, she said such constant flooding has meant communities have not gone back into a cycle of complacency.
She was a five-year-old living in Brisbane when the city was flooded in 1974 and remembers hosing down muddy linen from the homes of flood victims.
She said there was a perception in Brisbane that the Somerset dam, built in the 1950s, would mitigate any future floods.
"So I think people sort of felt complacent that we wouldn't get a severe flood again," she said.
Heading into 1974, the catchments were soaked, then 872 millimetres of rain fell on Brisbane in January, five times the monthly average.
"Basically it meant that most of the Queensland rivers were in flood, and so most of Queensland was submerged. And then just in case that wasn't enough, we got three huge cyclones," she said.
Following the flood, areas of Brisbane were declared no-build zones and some houses along the flooded creeks were bought by councils and not rebuilt.
"But that was a short-term scheme. Because again, what happens usually is we go back into the dry and those are put in abeyance because we stop worrying about the other climatic problems," she said.
Dr Cook said the risk of more flooding this summer should be taken seriously.
"We are actually getting some warning, so maybe there's a great opportunity to get some really practical things on the ground, getting some people out in advance, helping people to pack up," she said.
"We could be doing that in December and January."
Dr Santoso said a triple La Niña event was scientifically intriguing.
"But for society, the risk, especially to people who have been affected by flooding in past years, that is why it is not a good prospect," he said.
