Malcolm Turnbull has called the mass shooting in Las Vegas a "shocking senseless attack".
"Australia mourns with America tonight..." he tweeted shortly after the news broke on Monday night.
A gunman holed up in a 32nd floor hotel room at the Mandalay Bay Resort and Casino fired on the 22,000-strong crowd at an outdoor country music concert below.
Stephen Paddock, 64, killed at least 58 people and wounded more than 400 in the US' worst mass shooting.
Opposition Leader Bill Shorten described the scenes out of Las Vegas as horrifying.
"Australians holding our American friends in our hearts tonight," he tweeted.
It's not know whether any Australians were hurt in the attack.
"If you have any concerns for the welfare of family and friends in the Las Vegas area, you should attempt to contact them directly," a Department of Foreign Affairs and Trade spokeswoman told AAP.
People unable to make contact with loved ones should phone the DFAT emergency hotline: 1300 555 135, or +61 2 6261 3305.
Meanwhile, Las Vegas police said they had cleared Marilou Danley, 62, reportedly an Australian and the gunman's girlfriend, of any involvement.
