Your attention is drawn to the
ICAO PBN manual
On page 185
For RNAV 1 routes, pilots must use a lateral deviation indicator, flight director, or autopilot in lateral navigation mode.
On page 235
For RNP 1 routes, pilots must use a lateral deviation indicator, flight director, or autopilot in lateral navigation mode. Pilots of aircraft with a lateral deviation display must ensure that lateral deviation scaling is suitable for the navigation accuracy associated with the route/procedure (e.g. full-scale deflection: ±1 NM for RNP 1).
Your attention is drawn to
AIP SUP H05/18, which specifies RNP 1 for terminal operations in Australia.
Your attention is drawn to
CAO 20.91.
For RNAV 1 routes, a lateral deviation indicator, flight director or autopilot in LNAV mode must be used
For RNP 1 routes, a lateral deviation indicator, flight director or autopilot in LNAV mode must be used
A lateral deviation indicator must meet these requirements:
Pilots of aircraft with a lateral deviation display must ensure that lateral deviation scaling is suitable for the navigation accuracy associated with the route/procedure (e.g. full-scale deflection: ±1 NM for RNAV 1, ±2 NM for RNAV 2, or ±5 NM for TSO-C129() equipment on RNAV 2 routes)
If an aircraft does not have a lateral deviation indicator with a 1 mile scale (the A320 does not), suppose that leaves no choice but to use the flight director or autopilot.
Yes, sure, the MEL can allow you to fly without an autopilot and a flight director, but you will not be able to comply with RNP1. RNP 1 is a capability that would have been inserted in the flight plan and so you must comply. If the autopilot and/or Flight director fails, then you don't have any choice but you would also have to advise ATC of your reduced NAV PBN capability.