I'd go with Cambridge Gulf, on the north coast of Western Australia- Wyndham, and the Ord River estuary in particular. If you want to facilitate trade, make a lot of quick capital, build up an urban population fast by enticing in as many settlers as possible, there aren't gonna be too many places better. Main reason why? There's gold in them thar hills (along with an over-abundance of rare, precious rare-earth metals, zinc, lead and iron ore)- just follow the river up to Halls Creek. Along with plenty of pearls to be farmed on those coasts. And the biggest selling point of all, for settlers and prospectors in 1700- the diamonds. With a fair few diamonds to be found in the coastal waters of the Cambridge Gulf (the 'Argyle Alluvials'), having washed downstream from their source of origin, as well as in the Bow River Mine, but with the mother-lode located in the Argyle pipe further upstream; which just so happens to be the single richest diamond deposit in the world.
The world's largest volume producer of diamonds, Argyle's supplied a third of the world's diamonds every year since the late 1980s, with over 90% of the world’s supply of pink and red diamonds (along with roughly 80% of the world's supply of purple diamonds) having come from this single deposit, which contains a large proportion of the world's economic diamond resources, and has some of the highest diamond grades in the world. And Ellendale, a smaller diamond 'pipe' with an even higher proportion of gem-quality diamond than Argyle, and which produces intense yellow diamonds instead, can be found in the Fitzroy Valley of the west Kimberley region. If it has to be somewhere on the northern shore of Australia, to help facilitate trade with Southeast Asia, and your time-traveller wants to turn in decent profit margins, they'd be your best bets.