Doctors at Perth's new Fiona Stanley Hospital have been treating about 100 extra people every day at the emergency department than at the now-closed Fremantle Hospital.

The increase has come without an apparent drop-off at other emergency departments in the Perth metropolitan area.

Doctors are unsure where the extra patients are coming from, although some suggest they could represent unmet need from Perth's southern suburbs.

Fiona Stanley's emergency department head Mark Monaghan said they were all legitimate patients rather than people essentially wanting to have a look at the new hospital.

Fremantle's emergency department, which was closed last month, had received about 160 patients a day.

Dr Monaghan said the new hospital had been averaging more than 260 since it opened on February 3.

Almost 290 patients presented on the department's first Sunday, six days after opening.

"It's been a really challenging few weeks [but] the department's doing very well," Dr Monaghan said.

"The staff have worked enormously hard. We've had a very significant patient presentation load."

Staff surprised by high demand

He said the numbers had taken staff by surprise.

"I expected to go up to that [figure] slower than what we have.

"Certainly, by day six going to 290 was an enormous shock to us.

"I was hoping to have time that we could get used to our processes and make that a bit more gentle. It hasn't been that way, but despite that we're doing all right."

Department of Health figures taken during comparable weeks in January and February showed emergency department attendance at Sir Charles Gairdner Hospital, Joondalup Health Campus and Armadale Hospital, had either risen or remained steady since Fiona Stanley opened.

Figures from Royal Perth Hospital during the same period showed a drop of 150 patients over a week.

Dr Monaghan said it was not clear where the extra patients were coming from.

"We drain a lot of the south below us, so it may be, particularly with our paediatric area, that children are coming up more than they would have otherwise.

"We'll see over time. At the moment, it's just been a challenging start."

In a statement, the Fiona Stanley Hospital said many of the patients had been walk-ins rather than people brought by ambulance.

"Although the volume of presentations increased very rapidly once the department opened, the numbers are in line with what the demand projections had previously indicated," the hospital said.

No capacity for future increases: AMA

The Australian Medical Association's state emergency division spokesman Dave Mountain said patient numbers typically increased when large new medical facilities were opened.

He said Fiona Stanley was addressing an unmet need in Perth's southern suburbs.

"Unfortunately the modelling is always done on the presumption that everybody's needs have already been met," Dr Mountain said.

"Whereas we know that in the community there are unmet needs and people that are not getting full service, and whenever you open a new facility or a new hospital, you actually meet that demand."

With such high numbers already presenting at Fiona Stanley, Dr Mountain questioned how future needs would be met.

"There's recently published research suggesting that emergency attendances are going to continue to increase over the next four or five years at quite high rates," he said.

"So we need to be planning for the future, not just hoping that Fiona Stanley was going to solve all our problems."

Health Minister Kim Hames has been contacted for comment.