St John Ambulance WA has warned Perth residents to expect delays in response times as demand soars and ramping continues at hospitals.
St John issued a social media alert to say anyone calling triple-0 for assistance that they would likely experience a delayed response time.
It linked the delays to ongoing staffing issues and "extremely high" demand.
St John has also been grappling with the problem of ambulance ramping, and said roughly 22 per cent of the Perth metropolitan fleet was currently ramped.
So far this month, ambulances have been ramped outside Perth hospitals for more than 1,300 hours.
In a statement, St John said its State Operations Centre was receiving more than 40 calls an hour, while operating at about 7 per cent stand-by capacity.
St John is urging people to use other options for non-emergency care, including the Healthdirect telephone service or their local Urgent Care Centre.
It said its priority was to provide care to West Australians who required life-saving assistance.
The organisation said public messaging was part of its COVID-19 escalation plan.
"While we appreciate there will be a high level of interest in this messaging, our priority is to meet patient demand during this time," St John said in a statement.
Last month, Western Australia recorded its fifth-worst month on record for ambulance ramping, as the state's health system braced for a worse-than-usual flu season.
Figures for May, published by St John Ambulance WA, showed paramedics spent a total of 5,252 hours ramped outside hospitals.
It was the highest-ever total recorded for the month of May.
At the time, it took the total number of ramped hours this year to more than 24,000.
Problem due to staff illness: Government
In a statement, the WA government said St John had informed it the latest issue could be attributed to a staff shortage.
"Due to staff illness, separate to COVID furloughing, a shortage of available paramedics has significantly impacted their roster," the statement read.
The government said it was doing all it could to assist St John with this "unexpected" issue, including using career firefighters to support paramedics.
"Additionally, our hospitals have been notified of the situation and are doing everything they can to ensure ambulance crews attending hospitals aren't waiting there any longer than is absolutely necessary."
It also said ambulance ramping rates were lower than usual.
"Ramped crew levels are below the threshold at which St John escalates to a Status Level 2 alert," it said.
Warning follow deaths linked to ambulance delays
Also last month, West Australian Premier Mark McGowan apologised to the family of a woman who died while waiting more than two hours for an ambulance.
Georgina Wild died of a heart attack after initially calling triple-0 about 2:30am complaining of chest pains.
It is understood that St John checked on Ms Wild's welfare by phone twice, at 3am and again at 3:30am, advising her no ambulance was available yet.
She did not answer a third welfare check call at 4am when a call centre worker left a message on her answering machine.
Ms Wild was the third woman to die in a month after issues with the ambulance service, following the deaths of a Geraldton woman while waiting for an ambulance, and a Busselton woman who was in the care of paramedics when she died after waiting three hours for treatment.
