Naming conventions for RNP, RNAV, GNSS, etc approaches are a dog’s breakfast.
There is currently a process to rename all RNAV approaches to be RNP approaches. The old RNP approaches will be “RNP (AR)” (approval required).
While this is happening there will be a mix of RNAV, RNP, and RNP (AR). The best way to ensure you’re flying the correct approach is by the identifying letter, eg RNP Z or RNP M.
So in the case of Melbourne, if you’re cleared for the RNP Z and your charts only have the RNAV Z then that is the correct approach to fly.
Clear as mud.
Edit: Reading ICAO again, I see it’s even more confusing. RNAV (GNSS) becomes RNP and RNAV (RNP) becomes RNP (AR). Easy to confuse a newly named RNP approach with an older RNAV (RNP) approach but the latter requires specific operator approval.
There’s some more info here: https://www.icao.int/MID/Documents/2018/AIM%20SG4%20-%20MIDAD%20TF5/MIDAD%20TF5%20AIM%20SG4%20-%20PPT6%20RNAV%20to%20RNP%20Charting%20depiction.pdf