More than 40 firefighters were called in to fight the blaze, which is now believed to be under control.
Caller Deb told 3AW Afternoon’s Denis Walter hundreds of people are stuck waiting to leave.
“There’s been a car on fire opposite David Jones on one of the lower levels,” she said.
“But disruptions have been huge.
“They’ve told us the car park may not be open for another hour yet.
“There are literally hundreds and hundreds of people waiting, and when we try to get out there will be the mother of all traffic jams.”
Victoria Police say the exact cause of the fire is still to be determined.