For some months after we asked CASA to clarify this, we were of the opinion that "charter aircraft" referred to "an aircraft engaged in a charter operation".
The answer we were given - with specific reference to 20.18.10.1 - did not define the term, but was along the lines of (direct quote):
If a Charter aircraft is being operated on a private VFR flight, and an instrument . . . was to become unserviceable (and) the instrument is not a requirement for VFR, the aircraft can fly VFR.
In this instance, "Charter aircraft" refers to a "charter category" aircraft conducting a private flight.
However, if the definition of "charter aircraft" in 20.18.10.1 means a "charter category aircraft", then clearly the above aircraft could not conduct the flight - it could not conduct any flight with an unservicable instrument or equpiment.
So on the one hand, CASA has defined "charter aircraft" as being in the "charter category", and yet it has told us we can still fly private etc despite 20.18.10.1 - although, if they really wanted to get technical, they could point out the fact that 20.18.10.1 states the aircraft cannot take off (which is different to already being in the air).
Until recently this was all hypothetical. However, we are now faced with a situation that has led us to start questioning what we were originally told.