I have had this plant in a pot for over a year now, it has grown really well, flowered and I gave it a light prune to shape.

Now, it's turning brown.

What has happened?

What have I done?

A small "patch" is turning brown and now the tips are turning brown.

I don't want to lose it.

I am not familiar with Tetratheca in field conditions but quite possibly they die back to their root stock in drought and re-shoot once moisture levels improve. This is very common among a wide range of native plants.