The snub comes after the council agreed to review its culvert blockage policy, admitting it may have problems - after rigidly defending the policy throughout months of criticism from home owners and experts last year.

While council knocked back the engineers' offer, it has yet to work out who will conduct the review, its scope, or the timing, meaning home owners remain in limbo.

One of the engineers, Anthony Barthelmess, has been involved with Australia's leading research on the subject, and his work is internationally recognised.

The other two, Rob French and Paul Nichols, have years of experience in this field and have raised the blockage issue before. Mr French voiced his concerns about the drainage and blockage assumptions in flood modelling back in 2013, asking people to help collect evidence of flood blockage.

All three offered to work for free, going through the council's data and photographs from the 1998 flood, and giving advice on any conclusions that could be drawn, to help form a new policy.

The heads of council's flood and stormwater management section told them their assistance was not welcome.

They were told if they wanted to view the information they would need to submit a request under the Government Information (Public Access) Act.

Each of the three engineers had sought access to the documents - about 4500 photographs, and interview records - over the past five years, as they wanted to see the evidence on which the council's blockage policy was based.

They were told the information could not be found, or had been lost.

The documents have now been recovered, after Cr Greg Petty helped put the engineers' request to council.

Cr Petty said the refusal did not make sense.

"I feel the walls have been put up and the shutters slammed down watertight," he said.

"The information and background made available to me by interested parties is certainly causing me great concern when I see the responses from council.

"We have a genuine offer from three experts to catalogue all council's data and as a cash-strapped council we reject the offer. What sense does that make?"

Cr Petty said appointing an independent party to lead the review was still reliant on winning a state government grant for the work, meaning more delays.

"Where is the transparency in this process? There is none," he said. "Council needs to accept the offer because it is failing its duty to the residents - the consultation from the experts has been offered pro-bono and constitutes best value."

Council's manager of infrastructure strategy and planning Mike Dowd said the "scope and specification" of the blockage review was still being worked out, and a reviewer had not yet been chosen.

"The timeline will be determined when an appropriate independent body has been appointed but it is hoped the work can be carried out this year," he said.

"Council is seeking an appropriate industry-recognised, experienced and independent body to conduct the review.

"Council does not consider the individuals who made the offer as meeting these requirements."

Mr Dowd said much of the data had been provided under the Government Information (Public Access) Act request.

"We understand that the request has been approved and a significant amount of information has already been provided," he said.

"Any research that these individuals undertake will not be on behalf of Wollongong City Council."

At the heart of the issue are the culverts, drains and bridges under the railway line and the M1 Princes Motorway.

The council's blockage policy dictates that when modelling the flood risk for any area, any culvert or drain that is less than six metres across must be assumed to be completely blocked. This is the case for any significant flood, and is applied regardless of the characteristics of the site.

Drains are susceptible to blockage in large storms. But under Wollongong Council's policy, a 1.2-metre drain under the railway at Thirroul is regarded as being equally blocked as a series of eight large culverts, each 3.5 metres by 3.6 metres, under the freeway at Figtree.

Even the largest drains are considered to be 100 per cent blocked, as long as they are less than six metres across.

This, critics say, has had the effect of skewing the flood risk mapping undertaken for the council in several parts of the Wollongong local government area.

Residents in parts of Thirroul and Figtree have watched their insurance bills skyrocket as they are now classed as being in a high-risk flood zone, even though their properties have never been inundated.

The latest project in the Australian Rainfall & Runoff series produced by Engineers Australia, and regarded as the national standard, has recently been released after thousands of hours of research from experienced engineers.

The AR&R research was tested at several sites across Australia, including Wollongong, while the research conclusions were being formulated.

Two of the engineers (Mr French and Mr Nichols) have previously voiced their concerns about the council's blockage policy, saying it is too conservative and draws unfair conclusions.

The other one, Mr Barthelmess, has been a lead author on conduit blockage research for several years, including writing revisions to engineering standards which say a site-specific approach is needed, rather than a blanket blockage policy across an entire area.

Mr Nichols said the council's policy was based on a "worst in class" approach, but the AR&R guidelines predict a most likely blockage factor based on local catchment attributes, and historical evidence.

Both approaches were based on the same data.

"The difference is that the WCC policy uses a simplistic analysis that groups culverts on size of opening and then assigns a blockage factor that reflects the worst performing culvert in that class.

"The AR&R approach is based on far more sophisticated analysis, and considers likely culvert blockage factors."

Mr Dowd said the rebuttal of the engineers' offer was simply following proper council processes.

"Council procures the services of suitably skilled, experienced and independent consultants and scientific experts to provide advice or services to council.

"All of council's floodplain risk management studies and plans have been prepared by consultants who draw on the latest research, industry-endorsed standards and specialist expertise as required."