Luggage storage at MEL is available through Smartcarte. It's located in the International Arrivals area of the terminal. Prices are $10 for Hand Luggage, $14 for Suitcases and $20 for bulky items.
You literally walk out of Qantas arrivals and through some doors and take the escalators back up to International departures. The trip would be no more than 150m to transfer to DOM->INT. If you are arriving Internationally, then you come out on the ground floor,(where the baggage storage is) take the same escalators and walk straight ahead and you'll walk past the Qantas ticket desk in the domestic section.
Not sure about Virgin but should be very easy to find. Just follow the signs to International departures/arrivals.
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