The best solution for forwarding numbers isn't necessarily the best solution to text off. I think you may be better porting your mobile number to Google Voice and then forwarding the voice segment to a VOIP forwarding service. That way you get the text messages and voice calls. Also when you visit the US, you can temporarily reforward your Google number back to your US temporary SIM.

Another solution is to get a Truphhone SIM which includes Australia and the US on the same SIM, port your number, and get a dual SIM Android phone. Truphone isn't cheap enough to be your day-to-day Australian SIM, but it would neatly solve your problems in slot two.