THE State Government faces a $2 billion compensation bill if dam operators are found to have been negligent during the 2011 floods, one of the state's leading valuers says.

Iain Herriot, managing partner of WBP Herriots Queensland, said there were as many as 4500 "virtually unsaleable" properties in flood-hit suburbs of Ipswich and Brisbane.

At an average figure of $400,000 to compensate for loss of value and other damages, he estimated the total potential bill for the Government was more than $1.8 billion. That calculation did not take into account properties less badly affected.

"It's unfair that you should personally bear the cost and the loss that you may have occurred as a direct result of what may have been a result of operator error at Wivenhoe Dam," Mr Herriot said.

"If that is what the commission report finally reveals, the adversely affected owner has a classic case against the operator of the dam for compensation.

"If I had a flood-affected house ... the first nasty letter I would be writing would be to the dam operator and the Queensland Government saying 'give me the money - compensate me for your error'."

Mr Herriot said his firm, which mainly advises banks and building societies, had been closely monitoring the market in flood zones since January 2011 and estimated between 300 and 400 flood-hit homes had already sold in Ipswich at way below market prices.

"There was nothing for two or three months, then we started to see what we call 'remarkable' sales - they jump off the computer at you as being out of line," Mr Herriot said.

Owners needed protection from unscrupulous bargain hunters, he said.

"You're dealing with a distraught vendor and ... a greedy purchaser, so it does not meet the definition of fair value."

Ipswich Mayor Paul Pisasale said he was aware of "vultures" preying on flood-affected homeowners in his city.

He said real estate agents had told him investors were constantly asking about flood-damaged property in Ipswich.

"This city has been through an enormous amount of suffering ... and there are rogues out who like to capitalise on people's misery," he said.

Cr Pisasale said there was scope to better manage flood events but it was "for the inquiry" to determine whether there had been negligence or even political interference in 2011.

"These are the questions that should have been asked the first time round," he said.

A spokesman for Treasurer Andrew Fraser said the state "does not believe it will be, nor is it in possession of advice to suggest that it will be held liable" to compensate homeowners.

In any event, the Queensland Government Insurance Fund was "in place to meet all liabilities", the spokesman said.

LNP deputy leader Tim Nicholls said Queenslanders "may well look to the Government for recompense and that would be of great worry to Queenslanders given the debt and deficit situation we have at the moment".

Originally published as Flood compo bill tipped to hit $2b