A man was rushed to Liverpool hospital in a serious condition on Sunday after being mauled by three dogs in Sydney's south-west.

Police say the 49-year-old was running past a house on Maxwells Avenue at Miller when the dogs, believed to be bull terriers, gave chase.

He suffered multiple bite wounds to his face and arms before another man came to his rescue.

Witnesses say the man who tried to intervene was also attacked.

"The bloke got out of the bus and he's seen the dogs [biting] the man ... and he went and grabbed the hose and he went to give the dogs some water," the witness said.

"It didn't work out so he threw the hose away and the dogs went [after] him."

A spokeswoman for the NSW Ambulance crew confirmed the man who intervened required treatment for minor injuries.

A crime scene has been established and police say the dogs are likely to be destroyed.