For five days in April 1988, the Illawarra was drenched. The 600 millimetres that fell in less than a week was more than half the annual average rainfall.

The escarpment waterfalls were spectacular to see and hear through the mists.

The seas were wild.

The Illawarra is accustomed to extreme weather - wind storms, churning seas and saturating rain.

But by any expert and historic reckoning, this was a big event.

MORE: Engineer hits out at costly drainage rule

The tragic culmination - so well recorded and remembered by locals - came at precisely 3.23am on Saturday, April 30.

The Coledale railway embankment collapsed.

Terry Hagan had woken minutes before and went outside to move his car - a fateful decision.

Terry heard a loud crack and saw a mudslide come towards him at speed.

His house and family were swept away with no warning.

Wife Jennifer and son James were dead.

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Hours later, another house was swept off its foundations at Otford, north of Coledale.

Landslides were happening all over the place.

TWENTY-FIVE years on, there is still a question in some people's minds about the cause of this awful event. And still Wollongong wonders: could it happen again? What are the odds that more houses will be swept off the sides of hills?

Have our governments done enough to protect our homes? What else can be done to prevent another disaster?

The incident on that bleak night came under the scrutiny of the coroner and, since then, by water engineers - local and otherwise.

There has been some attention paid to the 1988 incident as well as the extraordinary flooding that occurred 10 years later in Wollongong - understanding the cause or causes could save lives and property when, not if, the next wild weather event occurs.

I am a water engineer by profession, who worked in Wollongong after the 1998 floods, with friends in Wollongong and a deep interest in the 1988 event.

It's important that the city of Wollongong and its 250,000 residents understand the risks of future disaster.

To determine the extent of the risk, we engineers need to accurately understand the causes of any extreme incident.

If we, as professionals, get it wrong, we could be wasting taxpayer time and money and fail to put in place infrastructure solutions that will actually save lives and property.

It's here that detailed investigative work is required.

From a working lifetime as a drought and flood engineer, the Coledale and Otford collapses do not appear to be flood-induced, erosional collapses.

They had all the hallmarks of ordinary landslides - caused by a long duration of rain that soaks into soil and fills up the little pore spaces with water under pressure.

The distinction between flood and landslide is significant. If a flood causes a problem, then drainage may be a solution - although not necessarily. If it's a landslide, a solution is problematic. Drainage will not stop landslides and getting insurance may be difficult.

Preventative planning action is required - do not build in high-risk areas.

I'VE researched the 1988 event in Wollongong City Council library - issues of the Illawarra Mercury and the SRA report.

The State Rail Authority investigation put the cause down to an unhappy conjunction of three things:

• "The long-term instability ... of the embankment [since its construction in the 1880s] ...

• the way in which the embankment was widened [in the 1980s] ...

• the blocked culvert [on the night of 29-30 April 1988]."

After long and persistent rain a landslide can occur at any time, whether it is still raining or not, whether there is surface run-off or not, whether it is flooding or not.

So what would a blocked culvert have to do with a landslide at Coledale? Here's the sequence of events early on Saturday as recorded in the SRA report:

• Just after midnight: The Sydney to Wollongong railway track 100 metres north of the Coledale platform had dropped a little and had moved half a metre out of line . . . the landslide was starting to move.

• 2.50am: The trains stopped running because there was no ballast or embankment visible under the rails and sleepers.

• 3.20am: There was water flowing from the north then to the east under the lines and sleepers, fast but not deep. Was this surface water that couldn't get down blocked Manhole No 1 near the upper end of the mudslide?

Or was it water that was reported bubbling up out of a sump (Drop Pit No 1) - about 30 metres farther north?

Or was it simply water on the western side of the tracks drawn to this area because the embankment had dropped enough so that it no longer acted as a low dam, which had been funnelling the water into the culverts?

Three days later, engineers used chemical dye to show the Manhole No 1 and the culvert below it were not blocked even though there were a few sleepers and some minor junk dropped down the manhole, presumably by local yobbos who continue to leave piles of rubbish on the flat area to the west and north of the train station.

So we have some evidence and opinion that there was a culvert blockage and some evidence and opinion that there was none.

In my opinion as a long-time flood engineer, I think the SRA took the wrong option.

The Coledale collapse had all the hallmarks of a conventional landslide. To my mind, it had been coming slowly for several days and accelerated during the night.

The embankment was moving hours before water was running under the rails.

Even if the culvert was blocked, it could only have brought the landslide forward by a few minutes or hours, and possibly killing more of the Hagan family.

WOLLONGONG property owners are currently facing significant increases in flood insurance.

It may be difficult to understand the reasons but these increases are an indirect consequence of the 1998 floods.

How? The single presumed blockage at Coledale in 1998 has led to the urban myth "explanation" for the widespread and severe flooding of 1998.

Insurers, being clever people, may now be adding blockage, heavy rainfall and flooding to the list of hazards that come with living under the brow of the Illawarra escarpment.

On the basis of 1998 presumed blockages, massive and expensive drainage upgrades have been recommended to Wollongong City Council by consultant engineers.

If some of the 1998 blockage reports turn out to be arguable, the council could well end up spending your rates on works of no real value to the public.

In the interest of clearly understanding the future risk to lives and property in this great city, I am continuing investigations into how much of the blockage reported in 1998 was real, and how much really imaginary.

But I need your help.

If you have evidence of culvert blockage west of Coledale train station in April-May 1988 or photographs of heavy debris masses in the mouths of culverts or bridges around August 17, 1998, you can contact me at robert.french@optusnet.com.au.