Krystal Cain disappeared when the car she and her father Lenny Cain were travelling in was swamped by floodwaters near the Burnett Highway at Booubyjan, in the Gympie region, in the early hours of Saturday.

The search for the teenager, involving helicopters, boats, trail bikes, 4WD vehicles and even people on horseback, entered its fifth day on Wednesday.

More news: Bendigo property market to stay steady in 2022 after strong growth in last 18 months

"We have grave fears but we still hold hope of finding this young lady," Police Superintendent Michael Sawrey said on Tuesday.

A GoFundMe page has been set up to aid the search efforts for Krystal and has raised more than $4300 in three days.

"Krystal is a loving daughter, sister, granddaughter, niece and friend with an incredible talent to sketch all things amima," the page said.

"She was swept away in a torrent of water whilst travelling to Agnes Water to see her grandparents. Krystal is still missing and we hold hope that she is still alive somewhere."

Mr Cain was rescued after farmers Ken and Julie Thompson spotted him clinging to a tree about 9am on Saturday morning.

Other stories:

The couple called emergency services and kept an eye on Mr Cain with the scope on their rifle until the rescue helicopter arrived two hours later.

Mr Cain was taken to Bundaberg Hospital and treated for pneumonia and broken bones. He has since been discharged.

The flooding has also killed two men and damaged dozens of homes and businesses.

Bauple farmer Steve Bottcher, 52, and a 22-year-old Sunshine Coast man have already been killed in the floods that came after ex-cyclone Seth dumped more than half a metre of rain on the Wide Bay Burnett region.

Water has begun to recede in the Maryborough area, about three hours' drive north of Brisbane, after the river peaked at just under 10 metres on Sunday night.

More news: Lifeline Central Victoria and Mallee to hold domestic violence awareness session to build skills in the community

A levee was protecting the CBD until an underground storm water mechanism failed just before 2pm on Sunday, allowing floodwater to surge up through the drains and into the streets.

More than 30 inner-city blocks were issued with an evacuation order after the remnants of tropical cyclone Seth dumped 600mm of rain on the Wide Bay-Burnett region in two days.

Prime Minister Scott Morrison said his thoughts are with the girl's family.

"I have a daughter who's 14 years old, so I can understand the terrible, terrible time that family must be going through at the moment," he said on Monday.

Mr Morrison also gave his condolences to the family of a 22-year-old man whose body was found in a submerged ute at Kanigan, north of Gympie, on Saturday.

-With Australian Associated Press

Our journalists work hard to provide local, up-to-date news to the community. This is how you can access our trusted content: