My perception is that a relief teacher is substitute teacher who continues teaching in the class till the end of the term, that means the previous teacher won't certainly come back, he is too ill tocome back. But if the previous teacher is going to back, then we cannot say the current teacher, is going to be called substitute teacher rather than relief.
The difference is mainly a regional one, not a semantic one--like "elevator" versus "lift." You need to specify which dialect you're interested in.
In idiomatic American English, any teacher taking over another teacher's class on a temporary basis is called a substitute teacher. In the UK, I believe the most common term is supply teacher (but a native speaker of that dialect should feel free to correct me). Relief teacher seems to be an Australian idiom.