Bystanders have rushed to try to save a man who they pulled unresponsive from a weir as firefighters used an inflatable boat to rescue his nephew from the water.

The man, aged in his 40s, is fighting for his life after the terrifying near-drowning at a Western Sydney weir on Friday.

The man and his teenage nephew were trying to cross Penrith Weir in Emu Plains when they got into trouble in the water about 10.40am.

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The uncle was unresponsive when he was pulled from the water, and bystanders performed CPR before paramedics arrived and managed to restore a pulse.

He was stabilised at the scene and taken to Westmead Hospital in a critical condition.

His 14-year-old nephew was stuck on the weir before being rescued by firefighters using an inflatable boat.

The boy was taken to Nepean Hospital with minor injuries.