Sydney couple's fight leaves baby injured

A baby is in hospital in Sydney's west after an alleged domestic violence attack that witnesses say escalated quickly and included threats to throw the child onto the road.

Police say they found the baby girl with bruising to her head and a man with injuries following reports of a couple fighting at a Parramatta unit complex on Tuesday afternoon.

Neighbours say police were called when they heard the husband and wife yelling and saw bottles thrown from level seven of the Great Western Highway apartment.

The couple were then seen arguing on the street, with one of them threatening to throw the baby onto the road, said a neighbour who witnessed the fight from his apartment.

Video footage taken by the neighbour, who did not want to be named, shows the man and woman arguing on the street as strangers, including a woman driving a truck, intervene and take the baby away.

"The lady stopped the truck and tried to help them and calm them down," he told reporters at the scene.

Police said a man in his 30s suffered bruising to his head and cuts to his body and a woman was arrested at the scene.

The unit's building manager said police and paramedics had been called several times in the past year because of the couple's fights.

"(I was) finding out what really was going on, trying to be the peacekeeper between the two," building manager Steffan Khatib said.

"Today unfortunately it has escalated, it has become (this) catastrophe."

Mr Khatib said he had seen injuries on the man before and advised him to call police.

"They've had ongoing domestic violence issues ... for approximately a year," he said.

The young girl was taken to the Children's Hospital at Westmead and is understood to be in a stable condition.

The woman is being questioned at Parramatta police station.

* National domestic violence helpline: 1800 737 732 or 1800RESPECT. In an emergency, call triple-zero.