Exclusive look inside Sydney's new luxury towers

It already boasts the record for Australia's most expensive residence; now One Sydney Harbour is heading skyward.

The first of three towers designed by Pritzker Prize-winning architect Renzo Piano is off the ground, keeping 1200 construction workers in jobs.

And off-the-plan sales for the apartments in Tower One have been so strong, defying recent market downturns, that 9News can reveal developer Lendlease is looking at bringing construction of Tower Two forward, creating even more jobs.

9News was given an exclusive look at One Sydney Harbour in Barangaroo South as it reaches its first construction milestone, with the tower rising past the ground floor and the core being poured to the underside of level five.

The penthouse set new Australian records when it sold late last year for more than $140 million.

The mystery buyer bought the two-storey penthouse and the sub-penthouse directly below.

The 1600-square-metre penthouse includes nine bedrooms, eight-metre high ceilings, a rooftop swimming pool, spa, gym, and a main bedroom the size of many Sydney homes.

Already, more than $1.5 billion worth of apartments have been sold off-market and the views are sensational, spanning the Harbour Bridge and Opera House and west across Darling Harbour to the Blue Mountains.

"In the end we'll have 72 levels, close to 250 metres tall and the tower will include 317 luxury apartments," said Lendlease head of residential Ben Christie.

Tower One is set for completion in 2023 with the overall project expected to finish in 2025 to coincide with the opening of the Sydney Metro.

Once complete, One Sydney Harbour will be the final piece in the award-winning waterfront residential, commercial, dining and retail precinct Barangaroo South, which normally hosts more than 20,000 workers and thousands of visitors each week.

It will neighbour Barangaroo's newest public spaces including Waterman's Cove, an amphitheatre-style boardwalk on the water, and Hickson Park, a one-hectare green link across Barangaroo South.

"One Sydney Harbour is ultimately going to need to live up to being the best residential apartments in the world," said Lendlease general manager Barangaroo South Jerome Johnson.

"In order to deliver that we'll have over 1000 workers from contractors across the industry to deliver the best of the best."

And with interest in the project defying the market downturn, Lendlease is now looking to fast-track Tower Two.

"From a buyer's perspective we could start selling those apartments by the end of this year," Mr Christie said.