A two-year-old girl has died in hospital after allegedly being assaulted by her mother’s de facto partner in Sydney’s west.

The girl was found unresponsive in a parked car at Guildford West on Monday afternoon, police said.

Local officers performed CPR before she was taken to hospital where she died last night.

Police arrested the mother’s 23-year-old de facto partner at Fairfield shortly after the attack.

The man was charged with reckless assault causing grievous bodily harm.

He is due to reappear at Parramatta Local Court next month and police said it was expected his charges would be upgraded to murder.