The main pre-European settlement contact with SE Asia was the trepang (sea cucumber) fishing industry. The SE Asian trepang trade was centred on Makassar in Sulawesi, and the early fishing areas were near Makassar. As the trade grew in the early 18th century, the Makassans went further in search of trepang, and collected them off West Papua, Timor, Sumbawa, and Australia. The trepang were processed ashore near where they were caught, and this meant contact and interaction with Australians when they were fishing in Australian waters. This interaction involved trade, cautious interaction, friendly interaction (with some Australians travelling to Sulawesi with the Makassans), and occasionally violence.

The Makassan trepang industry boomed in the mid-18th century (Schwerdtner Máñez and Ferse, 2010), and the established regular fishing in Australian waters by this time, by 1760 or so at the latest, and probably about 1720 (Macknight, 1986). Thus, this trade and contact began before European settlement of Australia. Large-scale trepang fishing in Australian waters doesn't appear to have begun before European discovery of Australia. For details about the dating of Makassan contact, and a profusion of further references on the topic, wee Wesley (2014).

Oral tradition records earlier regular visitors, people called the "Baijini" or "Bayini", apparently hunting whales and dugongs (Bowdler, 2002; Wesley, 2014). There was also earlier widespread trade in the area, which certainly reached West Papua, and might have reached Australia. The history of contact might go back millennia (Bowdler, 2002).

The other old trade route with Australia was across the Torres Strait, with the Torres Strait Islanders being the traders, exchanged good between Australia and New Guinea and Melanesia.

References:

Bowdler, S. (2002). "Hunters and traders in northern Australia". In K. Morrison & L. Junker (Eds.), Forager-Traders in South and Southeast Asia: Long-Term Histories (pp. 167-184). Cambridge: Cambridge University Press. doi:10.1017/CBO9780511489631.009

Macknight, C. (1986), "Macassans and the Aboriginal Past", Archaeology in Oceania, 21(1), 69-75. http://www.jstor.org/stable/40386713

Kathleen Schwerdtner Máñez and Sebastian C. A. Ferse, "The History of Makassan Trepang Fishing and Trade", PLoS ONE 5(6), e11346 (2010). https://doi.org/10.1371/journal.pone.0011346

Wesley, Daryl Lloyd, Bayini, Macassans, Balanda, and Bininj: defining the Indigenous past of Arnhem land through culture contact, PhD thesis, Australian National University, 2014, https://doi.org/10.25911/5c6e7069d3b9e