The man who died after his yacht capsized at Boat Harbour in Sydney's south had just bought the vessel.

The ABC understands the man was originally from Bundaberg, Queensland, and was in Sydney to sail the boat to Queensland.

A woman he was with swam to shore after the yacht overturned about 5:00am and notified paramedics.

It is understood the man was in his mid-70s.

A woman swam to shore this morning after the yacht capsized in Sydney's south. ( ABC News )

His body was spotted near Wanda Beach about 6:00am following a search involving NSW Ambulance helicopter and NSW Water Police.

He was brought to shore, but could not be revived.

The yacht, named C.C. Rider, had been docked at a wharf at Greenwell Point before the new owner picked it up yesterday.

The woman is in Sutherland Hospital being monitored, and an inquiry is underway by local police in Sutherland and the Marine Area Command.

It is unknown if either the man or woman on board were wearing lifejackets.

Acting Inspector Mel Thompson from the Sutherland Shire Police Area Command said the "seas were quite hazardous".

She said it was "remarkable" the young woman got to shore given the darkness of the early morning.