It is my understanding, that if "going through customs" is a requirement, it must be accomplished at the initial point of entry. Therefore, I am assuming that you MUST go through customs at the Sydney airport.
I will also assume that you probably won't have enough time to catch your connecting flight.
If you made the reservations yourself, you probably are "out of luck". If you used a travel agent, or another third party, then they should "adjust" your travel boarding to Melbourne (at no charge to you).
An hour between flights, especially a "long" first flight, is NOT a good idea. The first leg could be delayed, or the wind patterns might cause for a longer than publixhed flight time.
To land; taxi to the gate; leave the plane; get your luggage; get checked in to the next flight; board that plane; etc., in an hour usually isn't feasable.
At least you have time to change your ssecond flight.
Tony