After testing the modem in Bridge mode (2 devices can be connected) I quickly realised it was the modem that is an issue. In bridge mode the 2 connected devices work without any issues for any length of time. To further test this I then plugged in a Belkin Wireless Router (I had a spare Belkin N600 at home which I am using) and turned the Telstra modem back into the normal mode again. Since doing this and customising the config on the wireless router everything is working without fault.
BTW this is not a wifi related issue, if I plug ethernet into the modem I get the same issue just less frequently - plugging into the wireless router I have experienced no issues.
I'm not sure how to escalate further in Telstra - I have tried several routes all to no avail. The firmware is buggy as it is restricted to Telstra deployed modems and the vendor devices do not have the same issue reported.