Thousands of commuters have piled onto replacement buses on the first day of the closure of the T3 Bankstown Line, as development of the Sydney Metro extension towards the city's south-west commences.

Nine train stations along the T3 train line between Sydenham and Bankstown were closed this morning and will be shut for at least 12 months.

Southwest Link buses have replaced trains at affected stations in the inner west and south-west. The fleet, which consists of 100 pink buses, is scheduled to run every two-to-four minutes at peak times.

The initial reaction to the change from commuters was mixed, with some expressing frustration while others were more accepting of the situation.

'I'm hoping this will also be a boon for us'

Naseer Mohammed told ABC News that he was expecting peak hour traffic to slow his commute to Martin Place.

"Generally it takes 20 minutes from Bankstown to Sydenham, but going through Canterbury Road, it's going to be quite hectic," he said.

Mr Mohammed said he was looking forward to the line becoming part of the metro. He said it has made his commute into the city faster.

"It has decreased my time by another 10 minutes," he said.

"I'm hoping this will also be a boon for us."

Not everyone was so happy, with some labelling the first day of replacement services as "clunky" and "messy". 

"There was no true queue, so the bus pulled up and there was room for ten, and everyone just sort of ran for it," commuter Julia Tomlin said. 

At Bankstown station, one woman said she was "not happy" having to catch a bus for 12 months, but was looking forward to the metro.

"I go to the city everyday, and it's very convenient with the trains," she said.

"[Replacement buses are] not convenient, but it is what it is. So I have to live with it for one year at least."

Another woman said she didn't feel a replacement bus would disrupt her commute due to the frequency of services.

"It's more frequent than the actual trains ... I think it's good," she said.

"On this train line, there's always delays either at the station or stopping on the way to Central. And this is free, so I'm saving a bit of money."

The woman said she also "couldn't wait" for the metro.

"I hope it's not over one year. I hope it's as fast as possible."

'Real test' will be later in the week

Demand on the T3 line was lighter than usual on day one as it coincided with the first day of school holidays.

"The real test will come this week with Tuesdays, Wednesdays and Thursdays typically being the busiest on the public transport network," NSW Minister for Transport Jo Haylen said.

"Over time there will be busier days, we know for example when it's raining Sydney moves more slowly and this is a lot of buses — there will be congestion."

She thanked passengers for their patience and said it would be "worth it" once the metro was complete.

The T3 line has been closed to allow for the completion of the City and Southwest Metro.

Once complete, the metro will run from Sydenham to Bankstown alongside the T3 heavy rail, completing a 30-kilometre service between the north-west and south-west via North Sydney, the CBD and inner west.