Police say a woman whose car crashed into four people believed to be tourists in Sydney's north-west last night was drunk and texting on her mobile phone at the time of the incident.

Key points: Police allege the woman behind the wheel had a blood alcohol reading of .113

Police allege the woman behind the wheel had a blood alcohol reading of .113 The people injured are believed to be tourists from Hong Kong

The people injured are believed to be tourists from Hong Kong Police have urged witnesses, or people with dashcam footage, to come forward

Just before 9:30pm emergency services were called to Hezlett Road in Kellyville after the car woman hit four members of one family.

A four-year-old girl is in a critical condition after sustaining head and internal injuries. She was taken to the Children's Hospital at Westmead.

Her aunt, 55, suffered internal and spinal injuries in the crash and is in the Royal Prince Alfred Hospital.

The girl's mother, 44, sustained facial injuries and arm and pelvic fractures, and the girl's father also suffered non-life-threatening head injuries.

Lisa Mottram's blood alcohol level at the time of the crash was allegedly .113. ( Supplied: Facebook )

Lisa Mottram, 50, has been charged with three counts of dangerous driving, one of negligent driving, mid-range drink driving and using a mobile phone while driving.

Inspector Katie Orr from NSW Police's Crash Investigation Division alleged the woman's blood alcohol reading was .113.

"The police will allege that she was texting, sending and receiving text messages at the time," she said.

Police believe the injured family were from Hong Kong and in Sydney on holiday.

The woman has been granted conditional bail and will appear in Parramatta Local Court on October 10.

Police have urged witnesses or anyone with dashcam footage to come forward.

New penalties for using your mobile phone behind the wheel came into force in NSW today.

"The new increase has been from four demerit points to five so we are hoping it is a big deterrent because with the school holidays fast approaching and double demerit points that will double to 10 points which is close to a loss of your licence," Inspector Orr said.