At the height of the Lismore floods in February half the water in Sydney harbour flushed through the river system

There was also 80,000 tonnes of sediment that sloshed downstream.

That's the estimation of Southern Cross University's Professor Bradley Eyre, who has been has been chasing floods for more than 30 years.

But even he was surprised at the quantity of water that devastated the town of Lismore in the recent floods.

"At the peak of the first flood, the water coming down Wilsons and Leycester Creeks would have filled an Olympic-sized swimming pool in just over a second," he said.

"In total, around 250 billion litres of water came down these two creeks, which is enough to fill half of Sydney Harbour."

When extreme weather hit the Northern Rivers in February and again in March resulting in catastrophic floods, Professor Eyre jumped straight into action and was out on the flooded river, coordinating the use of the university's research vessels to rescue stranded flood victims (and their pets).

It also provided an unprecedented set of data for his current Australian Research Council (ARC) Linkage research project into the metabolism and greenhouse gas production in intermittent streams.

Out in the university's research-turned-rescue boats, extremely high concentrations of sediment could be seen in the water flowing through the creek system at the peak of the flood.

"An estimated 0.8 tonne of sediment per second was coming down Wilsons and Leyscester Creeks, which equated to an estimated 80,000 tonnes in total. Approximately 70 per cent of this sediment came through Leycester Creek which flows from the Tweed Range directly into the Wilsons River in Lismore," he said.

The second flood, occurring just a month later, saw approximately 50 per cent of the total volume of water compared to the first and an estimated 40 per cent of the amount of sediment.

According to Professor Eyre, these large floods are more likely to occur during wet La Nia years. These may increase in the future as climate change influences the frequency, intensity and impact of La Nia events. The latest report from the Intergovernmental Panel on Climate Change (IPCC) projected a doubling of flooding associated with global warming of 2oC this century.

As a coastal biogeochemist and researcher at SCU's Faculty of Science and Engineering, Professor Eyre has been sampling floods along the Queensland and NSW coasts and inland Australia for three decades. His first observations were conducted in the Richmond River, following a minor 1994 flood event.

His research has shown the catchment sediment and nutrient loads are mostly flushed out of the Richmond River Estuary during small and moderate sized floods. His work has also shown that landscape changes in the lower catchment, from wetlands to land uses with flood intolerant vegetation, causes deoxygenation in estuaries. "A comparison of deoxygenation events from 1940s to the 1990s also showed similar effects from small floods because many of the landscape changes such as draining wetlands for agriculture were made in early 1900s," he said.

Professor Eyre is currently leading four large ARC research projects, three of which are looking at the effects of extreme events such as floods on the aquatic biogeochemistry of intermittent coastal streams, tidal wetlands and inland dryland rivers.