Canberra Hospital was forced to divert ambulances to Calvary Hospital on Tuesday afternoon to ease pressure on its emergency department.
The department had record numbers of patients on Monday - more than 330 - and has reportedly been recording high presentations for a number of weeks.
Hospital bosses say there is no obvious cause for the dramatic increase in patients.
It's understood the hospital went on partial bypass on Tuesday afternoon, with non-critical patients diverted to Calvary Hospital for a number of hours.
It comes after junior doctors told The Canberra Times they had been facing increased workloads due to high demand in recent weeks.
Canberra Health Services CEO Bernadette McDonald asked Canberrans to go to walk-in centres or their GP if they didn't need emergency care.
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She said limb injuries, scratches or wounds could all be treated at walk-in centres.
"Go to the most appropriate place for care and you'll probably find you wait less time," she said.
Ms McDonald said the service was not yet aware what was causing the high demand.
The numbers are comparable to what the hospital records in peak winter periods and are unusual for November.
"It's putting a lot of pressure on our staff, but they continue to provide exceptional care to our patients," Ms McDonald said.
People were expected to face long waits, especially if they were non-urgent patients.
The hospital was forced onto bypass a number of times during winter last year, with authorities also at the time saying there was no obvious cause for the spike.
During a partial bypass, paediatrics patients, those with life threatening emergencies, and trauma patients are still taken directly to Canberra Hospital.
Stable patients will be diverted elsewhere.
Health Minister Rachel Stephen-Smith on Wednesday said high rates of mental health presentations could be partly behind the increased presentations.
There also appeared to be an increase in respiratory presentations.
"We've certainly seen a particularly bad pollen season and with the way that the weather has been behaving people who don't usually feel respiratory distress have been feeling that way," she said.
"That's probably party of the contribution."
Ms Stephen-Smith said about five patients were diverted to Calvary Hospital during the bypass.
She said there were a number of initiatives almost ready to come online - such as additional short stay mental health beds - which would help ease capacity issues.
However Canberra Hospital has been plagued for years with capacity and wait time issues, with the ACT consistently ranking as the worst performing jurisdiction in the country.
Opposition health spokeswoman Giulia Jones said it was just another example of capacity and workplace pressures at the hospital.
"The government continues to talk about their plans to improve the health system, but the evidence repeatedly shows this is not happening," she said.
"Labor and the Greens have been talking about expanding the Canberra Hospital for a decade, yet we are still waiting for the first sod to be turned."