More rain in recent years has helped cause the flooding which has plagued residents of the southern edge of Wollongong's CBD, the city council's flood consultant said.
Above-average rainfall, plus vegetation blockages, and changes to the lower end of the Gurungaty Waterway, were named as the major causes by consultants Jacobs Australia.
Jacobs was commissioned by Wollongong City Council to review the area's flood study, after several flood events have inundated homes and businesses near Swan and Kembla streets almost every recent year.
Increased concrete development and overflow from the Sydney Water sewage system were unlikely to significant causes of the flooding, Jacobs said.
These had been fingered by residents as potential causes of the persistent flooding that afflicts their neighbourhood, given the 1998 floods which devastated parts of Wollongong did not trouble the Swan St area.
Public consultation for the flood study returned figures that 19 per cent of residents of the area had experienced flooding on their property; 30 per cent in their streets.
But Jacobs found the sewer overflow contributed only six per cent to the June 2016 flood event, and three per cent to that in March 2017.
The firm analysed rainfall data from Port Kembla and found there had been more days with rainfall above 25mm, and above 100mm, in the past seven years than in the 10 years previous.
"Long-term, the number of these super-wet days per year was in line with the average since 1963," the report said.
The gauge at the Sydney Water treatment plant also showed there had been 10 rain events as heavy as that in March 2017 - 100mm - between 2003 and 2017, and none from 1998-2002.
"The wet weather overflows are not considered to be contributing factor to the flooding issues at Swan St and adjoining streets," the report says.
"The interaction between bridge obstruction and bridge blockages with creek flows increases flood risk at Swan St through backwater effects. The extent and degree of the increase in flood risk is dependent on a number of factors including the bridge geometry and degree of bridge blockages."
Some of the premises were built after 1998, and were not classed as being in a flood zone. They are now classed as being at significant risk.
Work to lower the Gurungaty causeway, to help flood waters escape, was to be completed this year, the report said.
The flood study will be open for public feedback until June 18, with information sessions at Wollongong Art Gallery, 4pm May 30, and JJ Kelly Park, 10am June 1.