A woman has been charged after allegedly attacking a man with a hammer while he was in a Sydney hospital bed.

Emergency services were called to Royal Prince Alfred Hospital in Camperdown about 12:15am on Thursday following reports of an assault.

NSW Police said Viki Graham, 46,allegedly entered the hospital and struck the man, who was known to her, with a hammer to his head.

The 63-year-old man suffered serious injuries and remains in a critical condition.

Ms Graham was arrested at the scene and taken to Newtown Police Station where she was charged with wounding a person with the intent to cause grievous bodily harm.

She was refused bail and is expected to appear before a bail court on Thursday.

Patient well known to staff

Sydney Local Health District chief executive Deborah Willcox said the patient was “well known” to hospital staff.

“We care about that person very much and our primary interest is his wellbeing,” she said.

“He is in a critical condition in our intensive care unit where we are taking very good care of him.”

Deborah Willcox speaks to the media.

Deborah Willcox says the patient is well known to hospital staff. (ABC News: Ethan RIx)

Ms Willcox acknowledged the woman who allegedly attacked the patient was given access to the ward outside of standard visiting ours.

“The staff did speak to the person who came into the ward and there was no issues [identified],” she said.

“There are standard visiting hours but this is a very big and busy hospital.

“We do whatever we can to enable people and friends and family to visit their loved ones.”

Ms Willcox said staff who witnessed and responded to the incident would be supported.

“There will be night duty staff who have gone home today to rest, they will not have to come back tonight,” she said.

“They did an outstanding job and you don’t come to work to expect this sort of thing but their training is so effective … that they knew exactly what to do.”

Ms Willcox said staff had attempted to make contact with the patient’s family and a review of the incident would occur.