here the biggest problem is the lack of agriculture and civilizations. Australia is really a poor place for growing crops, at least in the north where the Muslims would make first contact.
Yes, agriculture will be limited and difficult to take up in that region. But it's doable, tropical crops (banana, sago, etc., but not rice) can grow decently in those parts of OTL Northern Territory.
As has been mentioned, (and this subject has come up here before), the key is economics. Find something, whether it be gold, gems, or a rare plant that the aboriginees find and trade. Some enterprising Muslim traders from Southeast Asia/Indonesia might then set up a trading post, which grows into a settlement, which then spreads, coupled with some prosletyzing, and you could have a very large Islamic presence. What items are readily accessible in this way? Gold? Opals? I have no idea.
The diamond mines are a decent ways inland, so it would be by pure chance. Gold is mostly even further away from where the Indonesians visited, so that's probably out.
The best rare plant would probably be the
Tasmannia pepperbushes, which are a very good spice. Some of the
Backhousia myrtles could also be of interest. Unfortunately, they also don't occur anywhere near where Indonesians were.
I know most of the people were nomadic, but weren't there people who lived in permanent settlements and farmed in Northeastern Australia (I thought I read something about them from somewhere)?
Also, was sea cucumber really the only thing Australians had to offer when it came to trade? I feel like there is massive potential for a Kangaroo Fur Trade.
You might be thinking of the Gunditjmara, who were sedentary but non-agricultural (they did have a suprisingly complex system of eel aquaculture, however). And they lived in southeast Australia. I'm not aware of any other sedentary groups (there might have been some, though) and I know of none who farmed.
Kangaroo fur would be mostly useless, but kangaroo skin/leather makes for pretty good leather. Could be interesting, maybe set up a sort of tannery using Aboriginal workers. Probably won't go over well, however.