tell them the same. I flown Qantas, scoot, thai airways etc.. never happens. Not everyone is on annual leave. Maybe I'll fly home from vietnam/cambodia/Laos. Who knows? Once I was asked to show proof of accom. At immigration
Chances of needing to show ticket are very slim. Have you visited often? Do you overstay? I've never been asked and I've overstayed twice. IF the immigration officer asks, just say you will be catching a bus to cambodia, or buy a refundable airfare..