At least 75 patients were waiting for assistance when the computer assisted dispatch system suffered an IT glitch, ambulance union secretary Danny Hill said.
"What that means is they've called in, they require an ambulance, yet there is none available," he told 3AW radio on Friday.
"That's a sign of a system that's very much under strain when we've got that many cases pending."
The issue was resolved within 30 minutes, an Ambulance Victoria official said.
A 'code red' escalation was put in place as Ambulance Victoria and the Emergency Services Telecommunications Authority fixed the problem.
The computer assisted dispatch system pinpoints the nearest available ambulance and sends them to a case, Mr Hill said.
"When that crashes, they literally have to go to an old manual pen and paper type system to allocate crews and rely on the crews to actually manually call in over the radio to say 'we've just cleared at the Alfred Hospital'," he said.
"It's inefficient. It means that crews are often not the nearest crew that might be dispatched to a case and when we're busy ... you need every available minute and second to be saved and they can't do that when the system crashes."
It comes after a string of recent deaths in the state which have been linked to ambulance delays.
Last week, a budget estimates hearing was told at least 21 Victorians died over the past six months while waiting on ambulances, with a Melbourne law firm flagging a potential class action lawsuit.
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