Australia's most expensive residence, worth more than $140 million, will sit atop the One Sydney Harbour project after its developer Lendlease secured a buyer for the top three floors.

Yet to be built, the two-level penthouse and the sub-penthouse apartment in the tallest of the project's three-tower have been sold in a single deal to an Australian resident who will live there with his family, Lendlease confirmed on Saturday.