NSW Police have shot a man who ran at them with a knife outside a train station in Sydney's south west.

Key points: The man pulls a 30-centimetre knife on police once at the station and then later on a nearby street

The man pulls a 30-centimetre knife on police once at the station and then later on a nearby street Police use a taser but it is ineffective

Police use a taser but it is ineffective The man is currently at a hospital in a serious but stable condition

Transport Command officers confronted the man at Campbelltown train station after they noticed him acting suspiciously near the turnstiles.

He then produced a knife, leading police to fire a taser at him which was ineffective.

Assistant Commissioner Peter Thurtell said police then chased the man from the station to Hurley Street where he produced the knife again.

A blue baseball cap lies on the middle of the road next to medical equipment. ( AAP: Dean Lewins )

"He confronted police with a knife which has been described to me as about 30 centimetres long," he said.

The man, whose identity is unknown, is believed to have been shot in the stomach by police.

The shooting happened next to the busy train station as students from a nearby high school headed home.

He was taken to Liverpool Hospital and is in a serious but stable condition.

The shooting was captured by the train station's CCTV. ( AAP: Dean Lewins )

The shooting is being investigated by the Homicide Squad and overseen by the internal Professional Standards Command.

It is understood there will also be an independent investigation by the Law Enforcement Conduct Commission.

A street next to the station was taped off by police and medical equipment was seen lying in the middle of the road.

The incident was captured on the train station's CCTV.