Last month's big wet across much of the eastern Australian seaboard had few close precedents over the past 50 years and would have caused worse flooding had the region not been relatively dry before the event, the weather bureau said.
Ex-tropical cyclone Oswald dumped heavy rains from far north Queensland to the Hunter Valley in NSW, causing severe flooding in many areas within 200 kilometres of the coast.
"There aren't obvious previous examples of one that's tracked all the way down the coastal strip like this one," said Blair Trewin, senior climatologist at the Bureau of Meteorology.
Many communities continue to face disruption from the floods, particularly in the hardest-hit regions in Queensland, such as Bundaberg. The floods have revived calls, particularly from the insurance industry, for greater spending on levee banks and other works to protect towns from inundation. Claims in the state total some 28,000, for insured losses of about $320 million.
A dry spell over much of the region in the first weeks of January or earlier spared many areas from greater floods. Brisbane's catchment, for instance, copped slightly more rain over the wettest one and two days than it had during the city's 2011 floods.
"It was a really significant rainfall event in that catchment," Dr Trewin said. The big difference from 2011 was that "the lead-up period was quite dry", he said.
Bundaberg's Burnett catchment recorded the most exceptional rain, with rainfall averaging 204.1 millimetres on January 27, exceeding the previous record by almost 70 per cent.
It also broke century-plus records for rainfall over two, three and four-day periods by large margins, the bureau said.
In NSW, the Clarence catchment also saw record flood peaks, although towns, such as Grafton, avoided floods. Flood levels came within 2 centimetres of the top of Grafton's levee bank, a structure mostly dating from 1967.
While heavy rainfall is common from decaying cyclones, Oswald's extended southern path made its deluges unusual. A lingering high-pressure system over New Zealand delayed the normal east or south-easterly track of the storm until it had reached about Sydney, Dr Trewin said.
The last ex-tropical cyclone to stall so long in the vicinity of the coast was Cyclone Joy in 1990-91, which dumped some 2 metres of rain around Mackay over 10 days, spanning late-1990 and early-1991, he said.
The bureau has plans to release details of other severe weather related to last month's disaster, such high winds and tornadoes.
At least six tornadoes were reported over south-eastern Queensland during the storm.