They have not been completely isolated for 50,000 years, there has been several periods of limited contact.

The most significant is ~10,000 years ago was when Australia was finally culturally split from New Guinea, there is also linguistic evidence as 90% of Australian languages are within the same family and split only a few thousand years ago. However this is before the isopoint so not related.

What is important is genetic and trade evidence between India, South East Asia, and the northwest cost of Australia. This trade and gene flow occurred ~4,300 years and gave enough time for Australia and Tasmania to become completely mixed in the 1,000-3,000 years before the contact.

This is technically only evidence of India -> Australia but the evidence points towards continue contact and not a one off event. Continued contact points to the people returning from Australia to the homelands which allows for gene flow the other direction. It simply takes one person to make the trip and have descendants.

The dingo has only been in Australia for 4,000-10,000 years. If Australians have been isolated for 50,000 years where did this non-native animal come from?

Genome-wide data substantiate Holocene gene flow from India to Australia