Since originally asking this question I have now flown into Kuala Lumpur twice with a one-way ticket and into Singapore once with a one-way ticket.
Both times I flew into Kuala Lumpur, it was the final destination of my ticket. The first time I was asked for proof of onward travel when boarding in Australia. I showed the a printout of a train trip from Kuala Lumpur to Singapore which I believe I learned about due to the other answers here. The second time I had a photoshopped version of the same train ticket, only because the train’s website was broken! But I was not asked for proof of onward travel.
The time I flew into Singapore, roughly one year ago now, it was just a 4-hour stopover on my way to Taiwan. I was not asked for proof of onward travel when I boarded in Australia but I was asked when I was re-boarding in Singapore for Taipei. I showed them the flight booking I obtained for my Chinese visa and which I had subsequently cancelled.
So my advice is that there’s a good chance you might be asked at either airport, even if you’ve done it before and weren’t asked that time. So be prepared.
Johor Bahru is the first stop into Malaysia from Singapore, and is very easy to reach by ground transport. According to Seat61 the journey from Singapore to Johor Bahru is a whopping S$ 5, for the 5 minute journey across the border. Not sure you’ll manage much better than that for cheap onward travel proof!
I’d suggest you read through the Seat61 page on Singapore and Malaysia, then book a ticket online for the day you expect to leave. Show that at the border, spend a few days in Singapore, then head northwards by train. Quite possibly also carry on with your journey by train from there too, as it looks interesting from the photos!
I am most comfortable speaking from a US perspective, but I’ll assume MYS and SGP treat AUS the same or better. There do not appear to be any proof of departure plans required for either country for US citizens:
http://travel.state.gov/travel/cis_pa_tw/cis/cis_1017.html#entry_requirements
http://travel.state.gov/travel/cis_pa_tw/cis/cis_960.html#entry_requirements
I assume Australia has a similar resource somewhere on the Internet.
If I had to show proof of onward travel I would buy an inexpensive bus or airline ticket (as mentioned in a comment link above) and consider that to be the cost of my Visa stamp.
Culturally, Singapore would be more likely to enforce any Visa related rules, or any rules in general.
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