Sydney train commuters are being promised only minor disruptions as the city's busiest station undergoes "heart surgery" to make way for the new Metro.

The NSW government on Wednesday awarded the $955 million contract to transform Central Station, with construction to begin immediately.

Construction firm Laing O’Rourke, which won the contract, recently completed a $100 million upgrade of Wynyard Station in the CBD.

The upgrade, which includes new underground platforms and an underground concourse, is expected to be finished by 2022.

It will allow travellers to move easily between light rail, suburban and inter-city trains and the new underground Metro system.

“This is going to untangle Central,” Transport Minister Andrew Constance told reporters.

“We will, in essence, undertake open heart surgery on Central and we’re going to be able to do so without disrupting passenger services on a daily basis.”

Sydney Trains chief executive Howard Collins said the “challenging project” would benefit up to 400,000 people a day.

One platform has already been closed to allow work to begin and there could be changes to others.

“There will be some disruption, but the prize is worth waiting for,” Mr Collins said.

NSW Premier Gladys Berejiklian regrets there will be some temporary disruptions but argued “that’s no excuse for not doing it”.

“It would be negligent for us to sit on our hands and not improve Central railway station because it’s going to cause some interim disruption,” she told reporters.

Laing O’Rourke managing director Cathal O’Rourke said the company has developed techniques that will ensure minimal disruption during the four-year build.

“The station needs to be kept running, and its history and heritage protected, as we also upgrade it to the modern transport solution Metro offers,” he said.