How late can you really get to MEL airport for an early morning flight to Bali?

[dajop]

My flight was early afternoon to SIN. They had a BKK flight around the same time but different checkin area, so only accepting SIN passengers at the line I was queued in. One poor sod travelling to BKK had queued in the SIN line and they sent him away when he reached the counter! May be different first thing in the morning?