I guess "G'day" and "thanks" are Non-standard... in the sense that such phrases are not listed in the RT Phraseology... but neither are phrases like "do you have any weather on radar?", "that taxiway is closed" or "you're on fire!"
Standard phraseology is designed for set situations to prevent confusion and ambiguity... it is not meant to limit communication, or stiffle courtesy and politeness provided it isn't detrimental and used at the right time and place?
Good Day