Early days but the solution is indeed to block Google DNS on the Chromecast, however doing this properly is more of the problem since it is difficult to verify. For my Billion router the following are the correct (generic) instructions:
- Go to advanced routing or find where you can setup a static route.
- Destination IP: 8.8.8.8
- Subnet Mask: 255.255.255.255 or 255.255.255.252
- Gateway: Any valid local IP other than the router itself.
- Interface: LAN & Wireless
- Save & Continue
- Repeat steps 3 to 7 for the IPs of 8.8.4.4, 208.67.222.222 and 209.244.0.3.
Some sites recommend using the router itself as the Gateway, THIS DOESN'T WORK. In fact it shouldn't work since the router is the default Gateway anyway (so you are doing nothing new!). If you use a non-existent IP on your router LAN subnet then the Google DNS requests wil not get routed and Chromecast will fall back to the router default DNS (which should be the Telstra one).
This seems to be working for my Presto usage since Friday, at least for now.