The flood modelling method being implemented by Wollongong City Council has been described by senior engineers as too conservative and "overkill".
The council is being asked to change the way it calculates how culverts and pipes get blocked in floods, after residents and engineers criticised the skewed results the blockage policy produces.
Last month, the Mercury reported how residents of Northview Estate in Figtree were forecast to be inundated by water - despite never having flooded before - because the huge culverts under the M1 Princes Motorway were each less than six metres across.
The council's policy when mapping flood risk is to assume every culvert or pipe less than six metres is 100 per cent blocked. In effect, this turns the M1 Princes Motorway, and in other suburbs the railway line, into dam walls for the purpose of the modelling.
Modelling does not inundate a house or destroy property, but it has an impact on property values and use.
It has also caused insurance premiums to leap, from $1600 to $7000 a year for some in Figtree, and from $1300 to $8000 in Thirroul. The Thirroul policy did not include flood cover - that would take the premium above $20,000.
Illawarra-based engineers some years ago formed a group called the Association of Civil Engineering Consultants.
Civil engineer Glenn Mealey, the group's treasurer, said there was a widespread view in the industry that the 100 per cent blockage policy needed to change and engineers had argued this point before.
"To have 100 per cent was always far too conservative," he said.
"There's no other council that I'm aware of - and I'm 99 per cent positive - that uses 100 per cent blockage.
"They did it because of the possibility of people drowning and things like that, but they were being too conservative - worried about the council getting sued.
"If they've got a conduit that they're worried about because of blockage, they should be doing something about it.
"All [risk] is being thrown on to the residents."
Mr Mealey said there was a new Australian standard for blockage modelling, published in Australian Rainfall and Runoff, that was not yet ratified but could be used to provide guidance.
"There is no doubt there are flooding issues in Wollongong," he said.
"Because everything was developed holus-bolus over the years, there's lots of little blockage areas. But you don't need with that to be so conservative as well."
Opponents of the 100 per cent blockage policy have found support in the recent review of the Hewitts Creek flood study, being conducted for the council by consultants BMT WBM.
The new study found that BMT WBM's flood modelling around Woodlands Creek at Thirroul, sticking to the assumption of 100 per cent blockage, did not match the experience and records of residents in 1998.
But when the consultants remodelled this using zero per cent blockage instead, they found it matched 1998 well.
"The best correlation between modelled and observed flood levels was achieved with zero per cent blockages applied to the culverts at Princes Highway, the disused heavy vehicle safety ramp and the Illawarra Railway for this event," the report says.
Engineer Paul Nichols said this showed the 100 per cent blockage policy was flawed.
Wollongong City Council developed its blockage policy in the wake of the devastating 1998 floods, when debris blocking culverts was a major contributor to flooding.
It chose the six-metre threshold because analysis after the 1998 floods showed conduits larger than this did not get blocked.
Council's manager of infrastructure strategy, Mike Dowd, has previously told the Mercury the blockage policy could be reviewed if better science came to hand.
"There's a process we've had to apply [and] there are assumptions we've had to make based on whatever available information we've got," Mr Dowd said.
"That's what we're bound to in terms of the process, and that's the result.
"We acknowledge there's changes as a result of that, that changed risk profiles for individuals within those areas.
"We review it as and when new information evolves and new techniques come to hand.
"Until that comes to hand, we have to stand by the procedure we've applied."