The massive underground caverns for Sydney’s new metro railway station at Martin Place have been completed six months ahead of schedule.

NSW Premier Gladys Berejiklian and Minister for Transport Andrew Constance inspected the new Martin Place metro station on Thursday.

“In a few short years, Sydney’s new driverless trains will be running through the heart of the city every few minutes – a fast, new, reliable and safe railway extending from the Metro North West Line,” Ms Berejiklian said.

“This is an extraordinary milestone: excavation, tunnelling and caverns completed – next stop is laying tracks and building the new station which will service the heart of the Sydney CBD,” Mr Constance said.

It took nearly two years to excavate and build the mammoth caverns that will house the metro platforms.

The caverns under Castlereagh and Elizabeth Street are 28 metres below street level, 220 metres long and 14 metres wide.

About 126,000 tonnes of rock was excavated from the two caverns and more than 5500 tonnes of steel installed while the lining of the caverns required more than 21,500 tonnes of concrete.

Nine tunnels have also been built to connect commuters to the new station entrances as well as to the end of the platforms at the existing Martin Place Station.