While it's common to say the Illawarra was built on the coal industry, most of the time this is just a metaphor for the economic importance of coal mining and steelmaking for the region.
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But when Wollongong hydrologist Anthony Barthelmess says it, he means it literally.
Since the first coal mines began digging into the Illawarra escarpment, there had to be found some way to dispose of the waste from the extraction process.
Up until the last half a century, this coal wash was dumped, often with little controls over where the material might end up.
As the Illawarra grew in between the creeks, gullies and lagoons that characterise the coastal plain between the escarpment and the ocean, builders found a useful solution for the waste material.
Construction fill.
Now, what were once tidal lagoons and floodplains have been built over and stabilised, in some areas on up to 12 metres of fill has been used, particularly around the steelworks.
In a Wollongong City Council document from 1984, it was estimated that between 2000 and 4500 tonnes of coal wash was used each week in engineering applications, often in an uncontrolled manner.
Parks and ovals have been built on top of the fill and in some areas houses have been built along creeks and around lagoons filled in with coal wash.
Mr Barthelmess said that this past weekend as houses flooded and gushing torrents of rain washed cars away, Wollongong dealt with the consequences of this uncontrolled pattern of development.
"Back in the 60s and 70s, people had no experience with flooding, no comprehension of the way floods were going to occur," he said.
"We did a lot of historic filling with coal wash of lower areas of Wollongong that we would just never do today and back in the day, they were done and now people have homes in those areas."
Coal wash is still used as a building material today, particularly for the construction of roads, however much greater attention is paid to the material and how it is used, something that did not occur decades ago.
Coal wash is also particularly susceptible to moisture; the wetter the material gets, the more likely it is to break down. This may be part of the reason why when heavy rains in the region occur, not only are torrents of water channelled into creeks and gullies, it is those areas that are most at risk of washing away.
Given this, Mr Barthelmess said, it was "exceptional" that no one was killed during the weekend's flooding.
"Unfortunately, people now have homes in those areas and unfortunately when we have things like the weekend, those people suffer flood damages, and those old areas don't fare too well," he said.
"There have been some significant property damages, damage to the dwelling or loss of vehicles, and they seem to be isolated to those legacy areas."
Surveying the region as a whole, which endured double the monthly rainfall average in less than 24 hours, Mr Barthelmess said the lessons learned after the 1998 floods meant that new developments was insulated from the worst, and that it was legacy areas that bore the brunt.
