The Insurance Council of Australia says thousands of claims have already been lodged in the wake of flood damage in Queensland and New South Wales.
The council says insurers have fully staffed their call centres to deal with demand, but assessors will not be moving into affected areas until the waters have receded.
Council spokesman Campbell Fuller says insurers are hopeful the damage will be far less than from severe flooding two years ago.
"At the moment we're looking at about 6,500 claims having been received and estimated insurance losses of about $72 million," he said.
"Bundaberg is our main concern at the moment. We're aware of about 1,000 properties that have been inundated, and it's possible that this could double over the next few hours or days."
Mr Fuller says the damage bill is likely to rise considerably, but insurers were prepared financially and logistically for a wet summer.
"Many people won't have returned to their homes yet. We're still waiting to see what's going to happen throughout northern and central NSW - there are many areas of concern, including Grafton," he said.
"Bundaberg is our main worry at the moment, but of course there's other areas of Queensland where rivers are going to peak again later in the week."
Investors are concerned about the effect the damage bill may have on insurers' bottom lines, with the three major listed Australian insurers all falling substantially in a generally rising market.
Queensland-based insurer Suncorp has been hardest hit, with its shares falling 3.4 per cent to $10.54 by 11:08am (AEDT).
In an update to the market, Suncorp says it has received about 4,500 claims so far associated with flooding and storms generated by ex-tropical cyclone Oswald.
The company says it expects more claims to come in, with floods yet to peak in some areas.
However, Suncorp says it has set aside an allowance of $520 million for natural hazards this financial year, and such claims only totalled $147 by the end of December.
It also says its reinsurance program and loss sharing arrangements will help limit the financial impact of the floods.
QBE Insurance was down 2.5 per cent to $11.31, while Insurance Australia Group was off 2 per cent to $4.82.