A 39-year-old man has suffered serious head injuries after crashing his e-bike into a pole in south-west Sydney while allegedly riding away from police.

Police tried to stop the cyclist, who they said wasn't wearing a helmet, about 11pm on Saturday in Sadlier near Liverpool.

The rider allegedly fled from police but then hit a telegraph pole on the footpath.

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Police gave first aid but he suffered significant head injuries and was rushed to hospital.

He underwent surgery and is in a critical but stable condition.

The actions of the police officer are being investigated.

NSW Police Assistant Commissioner Brett McFadden said police understood the rider wasn't wearing a helmet.

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"One of the things we don't know is whether the person on the bike was known to the officers, whether there was a job they were responding to, whether there was intelligence they were responding to," he said.

"There's a lot of information that the officers have that may be able to assist with the investigation."

McFadden said he was unsure of what exact e-bike was involved in the collision, but told media it did not have pedals and was only powered by an engine.

Some such e-bikes are now illegal in NSW.

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"I can't give you an indication of what speed, but we do know the impact with the pole was significant," he said.

Despite investigations into the officer now under way the neighbours close to the incident that 9News spoke to have not been contacted by police, their security vision has not been requested and the officer behind the wheel has not been questioned

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