Thais visiting Australia on a Class 600 Visa needing onward flight.

Hi all, I have been searching for days on this topic, and can get no decent answer from the aussie gov or Jetstar.

My partner got her visa weeks ago and we are wanting to book flights now. 

The best so fare but not from any official is:

1/ Dont need but the airline might not let her board.

2/ Can board but arriving in Aus it is up to the officials.

anyone know the real answer?

Myself I doubt its needed. 

(reason we dont want return tickets is we dont know when we need return somewhere between 1-3 months)

Thanks to you all and keep up the good work.