Tens of thousands of Sydney commuters who travel by train between Bankstown and Sydenham each day will be forced to catch buses for more than six months while the line is converted to carry metro trains.

Transport Minister Andrew Constance said the government would try to minimise the length of time the 13.5-kilometre stretch of track would be out of action but conceded that it would "pose a challenge".

"It is going to be a disruptive time – I won't sugar-coat that," he said on Wednesday.

"There will be a period of time where that rail line will be decommissioned and as a result we are going to have to manage the commuter needs through that period."