I have ADSL2+ (telstra) and my wifi is through my modem
So your Telstra is also your also your WiFi and your router.
You have your Time Capsule Ethernet connected to the Telstra.
You should put your Time Capsule into "Bridge Mode".
Applications -> Utilities -> Airport Utility -> Your Time Capsule
-> Network tab -> Router Mode -> Off (Bridge Mode)
That way your Time Capsule is just a backup device on your network and not acting like another router.
Unless you also want your Time Capsule to also be a WiFi access point, you should turn off the Time Capsule's WiFi radio.
Applications -> Utilities -> Airport Utility -> Your Time Capsule
-> Wireless -> Network Mode -> Off
If you get PowerLine adapters, then just plug them into the Time Capsule's <--> Ethernet ports.
Here is a way to use PowerLine adapters to extend your WiFi. Plug the 1st PowerLine adapter into the Telstra. The put the other PowerLine adapter in the desired other part of our house. Plug the Time Capsule into that PowerLine adapter. Configure your Time Capsule to Bridge mode as described above. BUT keep the WiFi radio turned on, and give it the same WiFi network name the Telstra is using, and the same WiFi password the Telstra is using. Now you have set up what is called a "Roaming" network. This is NOT a Mesh network. Rather it is the poor cousin. But you should be able to have WiFi in 2 areas of your house. In addition, you can plug 3 Ethernet devices into your Time Capsule (your Netflix device for example).
If you are going to get a Mesh network, then if you are going to keep the Telstra as your router, you should see if you can turn off the routing capabilities in the Mesh network devices (the instructions for doing this will be different depending on which Mesh network you buy). Again you can plug the first Mesh network device into your Time Capsule. Then position the other devices around your house to give you better WiFi coverage.
If you get a Mesh network that has extra Ethernet ports, you can plug the 1st Mesh device into the Telstra and the Time Capsule into the extra Ethernet port on the Mesh network. But not all Mesh network devices have an extra Ethernet port so this is just a point of information.
Another way to approach this is to see if you can turn off the Telstra WiFi and Router functions, then make the first device that connects to the Telstra the router. If this is the Mesh network, then you would NOT put it into Bridge mode. If it is the Time Capsule, then you would not put the Time Capsule into to Bridge mode.
Basically, you have 3 potential routers, but ONLY would should be active. The Telstra, the Time Capsule, and the hypothetical Mesh network. If the Telstra is the router, then the other 2 should be put into Bridge mode (turn router features off). If the Telstra has its router functions turned off, then the device that is plugged into the Telstra should be the router. NEVER make the router the 3rd device in the chain. Things will not end well if you do that.
I know this s is a lot to consider, so if you just want to stop after putting the Time Capsule into Bridge mode and go with the Power Line adapters, I would understand. But if you do decide to get a Mesh network, and plug it into the Time Capsule, the Mesh network devices should also be put into Bridge mode. Things will really work better that way.