"Absolutely incredible" totals are set to continue as weather systems collide, bringing a peak to the rain overnight and into Tuesday.
Every state and territory, except Western Australia, is expected to be under some sort of heavy rain warning by this evening.
Very dangerous conditions continue for flood-affected areas as another 50 to 100 millimetres is expected to fall on already swollen catchments on Monday.
"We've got major flooding across the Hawkesbury, Nepean and Colo Rivers and moderate flood warnings to the mid-north, but we're expecting another peak today," said Jonathan How, senior meteorologist at the Bureau of Meteorology.
"So pretty treacherous conditions right along the coast."
On Monday another 50 to 100mm is expected across parts of Western Sydney, the Blue Mountains, the Hunter and into the Mid-North Coast extending up to Brisbane and the Gold Coast, with high isolated falls again.
The highest rainfall overnight was on the Gold Coast hinterland at North Tambourine recording 263mm. Brisbane CBD picked up 125mm.
In New South Wales, the highest was 245mm at Nambucca Heads on the Mid-North coast.
In the Blue Mountains the gauge at Kurrajong Heights recorded 174mm, 116mm at Springwood, 95mm at North Richmond, 93mm at Warragamba and 88mm at Penrith.
Sydney CBD only picked up 28mm in the end.
'Absolutely incredible' totals so far
The overnight totals were impressive but the totals since 9am Thursday were even more extreme.
Comboyn, just to the south of Port Macquarie, has recorded 889mm of rain between 9am Thursday and 9am Monday.
According to Mr How they were likely to approach 1 metre of rain over the next day or so.
The highest falls in the Sydney area over the same period was at Blackheath with 465mm.
In Western Sydney they recorded 353mm at Warragamba, 290mm at Penrith Lakes and 263mm at Richmond.
"So we're basically talking three to four months worth of rain over three to four days," Mr How said.
"Since records began, we haven't seen these types of numbers before."
The rain has not been restricted to just the last few days ? Yarras, also known as Mount Seaview on the Mid-North Coast, is up to 911mm for March so far and, according to Mr How,
"They could get to a metre over the next day or so, which is absolutely incredible."
Weather systems set to collide
From this evening it is about to get even wetter as a tropical cloud band swings in from the west, colliding with the coastal trough which has brought the rain so far.
"We've had days of torrential rain and now this tropical system coming through," Mr How said.
"It's a collision of two masses to create a huge rain/thunderstorm band that'll affect every state and territory except WA."
Overnight there have already been falls of 120mm across Central Australia.
Into Monday night, those heavy falls will push further east to places like Dubbo and in through the Northern Tablelands.
The resulting peak of the combined deluge is expected to be late tonight and into Tuesday morning.
"By tonight, we're likely to have every state and territory in Australia, except WA, under some sort of heavy rain warning," Mr How said.
Warnings are likely to extend to cover parts of the NT, South Australia, Queensland, New South Wales, the ACT, eastern Victoria, and most likely north-eastern parts of Tasmania.
"A huge section of Australia will be under this warning when the two systems collide tonight into Tuesday morning," Mr How said.
How unusual is this?
Weather systems colliding like this are not particularly unusual, but having this much rain in the lead-up is rare.
Mr How said that ironically for Sydney, the rainfall totals across the Hawkesbury and Nepean so far have not been more than what we saw in February last year.
"We did see a pretty similar event, in some regards, in February 2020, but the main difference was that coming off the drought, the soils were much drier and so had a much bigger capacity to absorb water," he said.
"This time around, unfortunately because of a wetter La Ni˝a summer, runoff is much greater."
Current analysis suggests that flood levels for this event will fall short of the 1867 flood event for the Hawkesbury and Nepean, but are likely to exceed the 1961 flood event.
While this event will be record breaking, especially for parts of NSW, it was not likely to provide much competition to the widespread rainfall totals set in 2010 and 2011 for Australia as a whole, according to Mr How.
While this rain will have been welcomed by some still needing rain, others, especially places north of Brisbane like Rockhampton, have again missed out.
When is it going to end?
The silver lining of the tropical cloud band is that it will push this coastal trough off into the Tasman Sea.
Despite a low pressure system embedding within the combined rainband rainfall is expected to clear up by Wednesday morning for most of NSW and Queensland.
Wet weather is likely to continue for Tasmania and Victoria through Wednesday but the system should be completely clear of the continent by Thursday, according to Mr How.
But because there has been so much rain, we are likely to see the rivers continue to fill well beyond Wednesday and continue to run high until the weekend.
"Even though the rain will stop and it will be mostly sunny in Sydney by Wednesday, the flooding risk will persist for some time to come," Mr How said.
© ABC 2021
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