Canberra Hospital was forced into ambulance bypass on Wednesday night with stable patients diverted to other hospitals.
Canberra Health Services says it is the third time this year the hospital has been on ambulance bypass, the other two dates being May 20 and July 1.
Health Minister Rachel Stephen-Smith confirmed during question time on Thursday in the ACT Legislative Assembly the emergency department was on ambulance bypass between 10pm and 2am.
She said stable patients that met clinically appropriate criteria had been diverted to other hospitals.
Ms Stephen-Smith said there was no reason for the increase in demand except for seasonal fluctuations.
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It's the latest in a long line of capacity issues facing Canberra Hospital.
Last month, The Canberra Timesrevealed beds had become a near permanent fixture in the emergency department's corridors.
Australian Medical Association ACT president Antonio Di Dio said the question of why the hospital couldn't cope with demand had to be asked.
"The fact that Canberra Hospital was on ambulance bypass for four hours last night gives us an insight into how busy our major public hospital is and the type of pressures that are placed on the medical, nursing and other staff," Dr Di Dio said.
"Of course, winter is a busy time - people get sick more often, seasonal illnesses such as the flu take their toll and even staff are sick more often. In short, the demands on the public hospital system increase significantly.
"It's core business for hospitals to plan for increased demand in winter.
"Additional beds are opened and staff are asked to fill in and work longer hours, more often.
"However, when Canberra Hospital is on ambulance bypass, regardless of the time of year, we're justified in asking - what's going on and why weren't we able to cope.
"A one-off event such as we saw last night can happen, but I'd start to be more concerned if we see a repeat of last night's bypass and particularly, for extended periods of time."
A Canberra Health Services spokesman said paediatrics patients, those with life threatening emergencies, and trauma patients are always taken directly to Canberra Hospital.
"[ACT Ambulance Service] never bypass a hospital during a life-threatening emergency," he said.
"To address the period of increased demand last night, the team at Canberra Health Services eased pressure across the hospital by creating internal capacity, discharging appropriate patients, and transferring suitable patients to private hospitals.
"A bypass provides a period of reduced inflow into the emergency department to ensure the emergency department stays safe.
"During a bypass, where possible, ambulances are diverted to an alternative hospital to manage the period of peak demand."
"We would also like to remind the community to only attend emergency departments in a genuine emergency.
"There are several options if you require non-urgent medical attention. These include the three Walk-In Centres located in Tuggeranong, Belconnen and Gungahlin. Additionally, you can also speak to your GP or other primary care provider."
Opposition health spokeswoman Vicki Dunne said the government was showing "its incompetence at managing the ACT's public health system".
"Last night's bypass is putting the safety of patients at risk and it means the hard-working and under-resourced doctors and nurses in our hospitals are coming under greater pressure," she said.
"We are a wealthy jurisdiction, with the most expensive public hospitals in Australia. The fact that the emergency department is in meltdown shows the Labor-Greens Government cannot manage health."