"It's on a knife edge which way it goes.
"But today is the day."
The bureau also has warnings for damaging winds, heavy rainfall and dangerous surf - Surf Lifesaving Queensland have already closed four beaches in Kawana, Maroochydore, Mudjimba and Dicky Beach.
The bureau is on high alert for flooding throughout the region.
Mr Marshall said thunderstorms were also a chance to increase the downpours.
It comes after widespread falls around the 20-30mm mark soaked the region since 9am Sunday with tops of 50mm at Mt Wolvi and 40mm at Tin Can Bay.
"The severe rain threat is today, yesterday was never going to be a big day," he said.