Hi guys,
Problem you have is with Wireless modem suplied by Telstra.It's not emiting strong signal as you need for these 3 walls.Ask Telstra to replace this modem for non-wireless Motorola 5100 and then buy powerful Wireless Router as Belkin , Lynksis or similar but MIMO or N , N1 series with strong signal out
http://www.belkin.com/flash/networking/ Other tips are if your laptop is not connecting wireless , un-plug and plug modem , sometimes they go to "sleep" and don't send new IP address when your lapy ask for one also changing channel on modem/router to Ch6 can help.
Hope this helped