From the IREX book: "For Charter and RPT operations the pilot must be able to suffer the failure of one airborne aid and still have a means of conducting an instrument approach."

The reference to this used to be AIP ENR 1.1 para 11.7.3.1 b.
Where is this reference now?

OK, so I have found a possible reference from which one could infer the idea of being able to endure the failure of one instrument approach aid in Air Transport (previously Charter and RPT) operations:

CASR Part 135 MOS

11.09 Navigation equipment

3)  Subject to subsections (7)* and (8)*, and without limiting the requirements under subsections (5) and (6), an aeroplane for an IFR flight must be fitted with at least:
(a) 2 approved GNSS;
or

(b) 1 approved GNSS and either:
(i) 1 ADF; or (ii) 1 VOR.

and

(5) The navigation equipment fitted to an aeroplane must be such that, in the event of the failure of any navigation equipment at any stage of a flight, sufficient navigation equipment remains to enable the aeroplane to navigate in accordance with:
(a) the aeroplane’s operational flight plan; and
(b) the requirements of:
(i) ATS; and
(ii) the airspace in which the aeroplane is planned to be flown.

(6) For any aerodrome at which it is planned, or intended, that an aeroplane may land in IMC, the aeroplane’s equipment must be capable of providing guidance to a point from which a safe visual, or instrument, landing may be conducted.

*subsection (7) says if using two GNSS they can’t be TSO129, and subsection (8) says if using only one GNSS it can only be TSO129 if made and installed before 6 Feb 2014.

In contrast, the CASR Part 91 MOS reference about requiring alternates (if PVT or AWK IFR, presumably) says:

CASR Part 91 MOS

8.05 Destination alternate aerodromes — navigation
(1) The pilot in command of an aircraft must nominate a destination alternate aerodrome if a flight is an IFR flight by night to a planned destination aerodrome that is:
(a) not served by an IAP; or
(b) is served by 1 or more IAPs none of which the pilot in command is able to conduct.

This is mirrored verbatim in the AIP ENR 1.1 10.7.3.1

For what it's worth, regarding the PVT IFR navigation equipment requirements, there are also references here:

CASR Part 91 MOS

CHAPTER 26 EQUIPMENT

26.07 Aeroplane VFR flight by night
(1) An aeroplane for an VFR flight by night must be fitted with:
(a) an approved GNSS; or
(b) an ADF or VOR.
...
26.08 Aeroplane IFR flight
(1) An aeroplane for an IFR flight must be fitted with the following navigation equipment:
(a) ...if aeroplane manufactured on or after 6 February 2014 — at least 1 approved GNSS but not one authorised in accordance with (E)TSO-C129;

(b) ...if aeroplane manufactured before 6 February 2014:

(i) ...GNSS installed on or after 6 February 2014 — at least 1 approved GNSS, but not one authorised in accordance with (E)TSO-C129;

(ii) ...GNSS installed before 6 February 2014 — at least:
A. 1 approved GNSS, but not one authorised in accordance with (E)TSO-C129; or
B. 1 approved GNSS that is authorised in accordance with (E)TSO-C129, and an ADF or VOR.

...

Thanks Rubber for your excellent reply.

Bob