Sydney airport is trickier than some airports. The domestic and international terminals are (iirc) 2km apart, so it's not like you can just hop over to the next terminal in a row. There is a shuttle. https://www.sydneyairport.com.au/info-sheet/get-to-your-next-flight
As far as Customs -- the queue will vary with time of day and how many flights are arriving close in time to yours. Probably your best option is to be intentional with your packing so that you will have nothing to declare. If that's not possible, declare everything. They don't usually need or want to pick through all your things; they will ask you questions and then either scan the bags, ask you to show them the things you're declaring, or just send you straight on your way (depending on what you have).
Food is the main thing. There are many trash bins between your arrival gate and Customs, specifically for the purpose of disposing of food you can't or don't want to declare. Get rid of anything homemade, open packets, fresh produce. (Sealed, store-bought processed food is generally fine.)
If you have bags to collect and re-check, your timing might be too tight. Baggage claim can be slow at any airport, and so can check-in/bag drop. If you're just travelling with carry-ons, it's probably doable.