The 13th currently has those four flights scheduled with the A330.
With 3x reward seats you probably will find it difficult to secure an alternative flight on a B737 closer to the date as this is 1/4 of the cabin, especially if you are disrupted on the day. It is also unlikely that the A330 from MEL to PER will be your arriving aircraft from SYD as 30mins is too short to turn it around. Recently I flew into SYD with a 45min transit and as my arriving flight was taxing to its parking bay I passed the gate where my onward flight was due to depart and noted to the cabin staff that my plane wasn't there yet and it is due to leave in 40mins. They said that was very unusual as it should be there at least 40mins and as it turned out it was but I was looking at the wrong gate.
With my experience of J on the A330 flights between SYD and MEL in the past year I wouldn't look forward to it too much and you would probably have a better experience in J on the B737 because the flight is short, on the A330 there are two aisles and lot of seats to service and you get very little attention during the flight. I have found the A330 has poor and limited choices of food and wine, wifi doesn't work (if available) and you don't have time to watch a movie. My most recent flight I didn't have a drink at all on the flight because it took so long for the cabin staff serving my aisle to come back and remove the horrible grenache that was given as the only red that she took it away without offering another - even though I asked as she was turning away - yet the other aisle was being served seconds and thirds. As you are on the A330 from MEL to PER that would be a more enjoyable experience of the wide body than SYD to MEL.
A fair portion of the flights running earlier than 17:30 have 3x J seats available at this time (16:00 already gone) but are limited and probably will disappear closer to April especially as this date is approaching the Easter holidays. I would suggest you look at moving to an earlier flight on a B737 to ensure you make the connection and enjoy the A330 on the longer leg.