Entry into Australia does not need the six month validity on your passport (
https://www.passports.gov.au/Pages/f...ions.aspx#faq8), but entry into Singapore does (
https://www.ica.gov.sg/page.aspx?pageid=95).
I realise you did not technically enter Singapore, but the airline is not able to stop you if you tried to enter.
Also, I believe the airline gets penalised by the destination country if they carried over passengers with improper documentation. I had to wait at the check-in counter a few times while phone calls were made to confirm that my documents were sufficient.