A gas bottle has fallen from the upper floors of a building in Sydney's CBD before striking and killing a man in a "freak accident" amid wild weather.
The 37-year-old victim was walking with a friend in The Rocks after work when the bottle struck him in the shoulder before midnight on Tuesday.
The man's lungs collapsed and he went into cardiac arrest.
"Traumatic cardiac arrest is serious and it's rare for someone to survive," NSW Ambulance Superintendent Kath Rallings told reporters on Wednesday.
""This is a freak accident and a tragedy for this man's family and friends."
Supt Rallings said the man's death was a "one-in-a-million" event.
"The man was unconscious when paramedics arrived and he then went into cardiac arrest, she said.
"While this appears to be a one-in-a-million accident please do not underestimate these wild-weather conditions."
Detective Inspector David El-Badawi says the gas bottle had fallen "from a great height" to strike the married Chatswood man and they are investigating where it had fallen from.
"The bottle hit him in the shoulder," Det Insp El-Badawi.
"It's a nine-kilogram gas bottle. We believe it has come from a nearby apartment building but we don't know that yet.
"If anyone is missing a gas bottle, we just need to know the circumstances behind this man's death, it's a tragic accident."
Detectives visited the scene outside the Malt Shovel Taphouse on Wednesday morning.
Det Insp El-Badawi said the man's friend was traumatised after witnessing the "freak incident".
A crime scene has been established on Harrington Street and investigators have appealed for anyone with information to come forward.
© AAP 2020