ACT health authorities say people with non-urgent problems should stay away from the Canberra Hospital's emergency department, or face a four to six hour wait.
Canberra Health Services on Tuesday issued a reminder to Canberrans on Facebook, saying "the ED should be for genuinely urgent matters" only.
The health service said the Canberra Hospital was under pressure.
It urged people with non-urgent problems to head to one of four walk-in centres around the territory, or to contact their GP or other primary care provider.
The walk-in centres are at Tuggeranong, Belconnen, the inner-north and Gungahlin.
Ambulances were not being diverted from the Canberra Hospital on Tuesday.
Canberra Health Services' chief executive Bernadette McDonald said early on Tuesday authorities couldn't be certain why the hospital was so busy.
However, she said there had been a slight peak in emergency surgery.
"As happens in hospitals, some days and some weeks are extremely, extremely busy, so we just put out a bit of social media to tell our community that if you don't need emergency treatment ... we do have other options," Ms McDonald said.
"The hospital is extremely busy today in particular and as we get a build up of people coming through, we need to discharge people.
"We just have opened extra capacity today and we're just trying to keep everyone safe and keep the flow going through the organisation, especially through the emergency department."
Ms McDonald said staff at Canberra Health Services were responding to the situation well and she was extremely proud of all employees.
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