For the record, Jews did migrate to India. I am not aware of the numbers, but they have been here since the first temple was destroyed around 586BC and the subsequent exodus of Jews from Palestine. Many more including St. Thomas moved to india after Jesus was executed and the romans put down the revolts.
Bonus fact, western coastal india where the jews settled is one of the few places on the planet where jews were historically not ostracised or persecuted. Well, at least until the Portuguese showed up.
Another fact about the Roman-Indian trade, the Romans traded extensively and directly with Indians. Berenice on the Red Sea was an important port from which around a hundred ships sailed for India every monsoon. Possibly there were roman settlements on the western coast of india.
As for australia, well, India and Asian traders probably knew about australia and had even interbred with the aborigines. This is not a fact i pulled out of my hat, but rather is a result of recent genetic analysis of Australian Aboriginals.
So it may not be completely unrealistic for a group of people from asia to make there way to Australia. People did that on rafts just a few thousand years ago. On a large Roman ship, following the monsoon trade routes and taking adequate stopovers, it is seems quite a lot more possible. They could potentially 'island hop' all the way down to Bali and Sunda on well established and mapped trade routes. From there all it will take is a storm to push them south into the northern territory, Gulf of Carpentaria, or northern part of western australia.
Of course all the other points raised about the technological stagnation and culture etc., might still be true. Also northern australia is not the most likely place to colonise. I mean it's kinda hostile with the flora and fauna and cyclones/typhoons.
Who knows, maybe a bunch of them did make it but died out and now their remains are buried under a parking lot somewhere in Darwin. (jk

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As for trade, well it's just 1800 km or something from Darwin to Bali, an important centre of trade. It is it inconceivable that jewish settlers who settled in northern australia will be completely cut off from mainland Asia and the spice trade networks? Maybe technological stagnation is not such a certainty.