HUGE areas of Gympie will become an inland sea when the long-overdue 1 in 100 year flood finally hits, a new regional flood study has revealed.
The comprehensive study estimates where the flood will spread if the Mary River peaks at 25.22m at Kidd Bridge.
It also predicts what will happen to properties near streams, creeks and gullies in rural residential developments and towns.
The last review of flood lines 12 years ago raised Gympie's 1 in 100 line significantly, putting many new properties under water in a massive flood but the maps adopted this month have barely changed within Gympie.
It is a different situation near Goomeri, Kilkivan, Imbil, Widgee and the Cooloola Coast which, until now, have had no official flood lines to guide development.
Councillor Larry Friske, who holds the water portfolio, says there are "no great surprises" from the study but some new home owners may be about to discover they have built below the flood line.
No official count has been done of the homes that will go under.
One immediate outcome of the study however has been to raise minimum floor levels on new homes at Tin Can Bay and Cooloola Cove by 0.6m and 0.8m respectively.
This was triggered by the potential threat of saltwater flooding on the coast from storm surge and climate change. Rainbow Beach and Curra flood lines are still under review.
It has been almost 120 years since Gympie had a 25-plus metre flood: the Mary River peaked at 25.45m in 1893.
The 100-year flood level means there is a one in 100 chance of water reaching that height in any year. During hundreds and hundreds of years, that would average out to once every 100 years.
The new flood lines will be available digitally for public perusal from next week at Gympie Regional Council's design department in the Old Post Office building in Duke St.