St John Ambulance recorded its busiest day on Monday, responding to almost 850 emergency call-outs across the State.
The St John metropolitan division, operating from Two Rocks to Pinjarra and east to Mundaring, also had its most prolific 24-hour period on Monday with 688 dispatches.
More than half of the metro calls were classified as "urgent", including about 240 life-threatening or priority-one incidents.
The average number of metro daily call-outs last year was 554 but the 650 mark has been passed three times this month, including on Thursday.
Metropolitan Ambulance Service general manager James Sherriff said a late start to the flu season had led to a surge in call-outs this month.
"The flu call-outs vary - some of them are priority one because flu exacerbates airway diseases, asthma and heart conditions," Mr Sherriff said.
"We find a lot of those flu patients have other conditions, then they do become life-threatening or higher priority cases."
On any given day St John will have 70 ambulance crews on metro roads and up to 40 at night.
Mr Sherriff said extra crews were placed at strategic locations during Monday's peak demand.
The triple-0 call centre in Belmont also called in extra staff.
Mr Sherriff said St John and Perth hospitals were trying to best determine where patients should be sent. "As the demand is increasing, we have to make sure we keep the pressure off the emergency departments as best as possible," he said.
"St John has a target of reaching 90 per cent of high-priority unscheduled events where patient's life is at risk within 15 minutes of receiving a call.
Mr Sherriff said response times had improved in each of the last four years, with 93.2 per cent of life-threatening calls in 2013-14 being attended to by an ambulance within 15 minutes.