Two-year-old left critical after allegedly being beaten by a Sydney man dies in hospital overnight.

A two-year-old girl who was assaulted by a 22-year-old man on Monday has died in hospital overnight, Nine News Australia reports.

It is expected that the charges laid against the man, who allegedly abused the child, will now be upgraded to murder.

On Tuesday the man was revealed to be 22-year-old Mohammed Khazma. Khazma had only known the girl’s mother, also aged 22, for a short time – about four to six weeks.

Police allege Mr Khazma physically assaulted the girl on Monday afternoon in a granny flat in Marian Avenue, Guildford, that the trio lived in.

Mr Khazma did not appear at Parramatta Local Court on Tuesday but was legally represented, the Sydney Morning Herald reports.

He did not apply for bail on the charge of reckless grievous bodily harm and is scheduled to face court again in January.

The relationship between Mr Khazma and the child could not be detailed for legal reasons.

It is understood the mother witnessed Mr Khazma allegedly physically assault the child and was able to intervene and leave the granny flat with the toddler in her car.

She then drove to a Western Sydney home shortly after the attack with her daughter in the back seat, neighbours have told Channel Seven News.

"Your son has killed my daughter," she reportedly yelled at the house's occupants.

When the woman collapsed in the front yard with her daughter unconscious in the car, family members from the household called emergency services to the Guildford West home on McCredie Rd.

A neighbour said she saw the mother appear at the front of the house.

"She (the mother) said, 'Your son hit me, your son has killed my daughter'," the neighbour said.

"I’ve never seen anything like it. I thought it was a big fire and that fire engines were going to show up but suddenly the street was full of cops."

Crime scenes were established at both addresses after forensic units were called to examine the properties.