A mother died and her baby daughter was in hospital in a serious condition after a swarm of bees attacked the pair outside their Sydney home yesterday afternoon.

Neighbors told police the 38-year-old woman was playing with her one-year-old girl when "something" caused the bees, kept in a hive at the side of the house, to swarm and attack.

"Neighbors described the pair as "covered in bees", said the duty officer, Sergeant John Stapleton, of Newtown Police.

He said it appeared the woman tried to run inside but collapsed and died at the front door after being stung.

An ambulance officer said the woman's daughter, screaming and struggling for breath, stood at her mother's feet - she had been stung more than 50 times.

The baby was rushed to Royal Prince Alfred Hospital and was later flown to Westmead Children's Hospital for treatment, an ambulance spokesman said.

A bee keeper called to the house by police reported finding eight hives, each with up to 80,000 bees. He told police the hives appeared to have been established for up to 10 years.

After inspecting the hives, the bee keeper opted to leave them overnight.

Police warned neighbors on each side of the brick home to stay inside and they blocked off the street for more than three hours.

Cases of bee swarms attacking people were "very rare", an Australian Museum scientist said last night.

But Dr Graham Pyke, a principal research scientist with the museum, said introduced honey bees could become very aggressive and attack people disturbing their nests.

"They are social," he said. "If disturbed they will swarm."

Dr Pyke said introduced honey bees, first brought into Australia from Europe about 160 years ago, were happy to set up nests around homes - sometimes even in the space between walls.

When bees kill it is usually because of anaphylaxis, a severe and sudden allergic reaction that triggers swelling in the throat and lungs.