Well I just spent an hour revieweing the ACMA website regarding CB radio Note the second paragraph below:
https://www.acma.gov.au/Citizen/TV-...io/citizen-band-radio-service-cbrs-fact-sheet
"Recent changes to the Citizen Band Radio Service
CB radio users will no longer have to replace their older-style 40 channel equipment. A limitation of the equipment’s use was due to take effect from 1 July this year. However, following consultation, the ACMA has decided to allow continued use of 40 channel equipment alongside the newer 80 channel equipment.
The limitation on interlinking repeaters through CB stations that previously applied has also been removed. The ACMA has identified the potential for the interlinking of CB repeaters to provide significant benefit to CB users, particularly in regional and remote areas. The ACMA has come to the view that these benefits outweigh any potential interference, and that alternative regulatory mechanisms are sufficient to address interference if it arises. "
While this seemingly addresses repeater linking, there is no stated prohibition for remote base operation in the current rules.
A further read indicates that specific channels are reserved for repeaters and that apparently a 75 KHz spacing is used. This is a bit narrow, but likely the low power nature of the service permits such close spacing.
https://www.legislation.gov.au/Details/F2017C00476
I would suggest that upon review that it would be legal for the OP to construct a remote base using a UHF CB station on channel 25 controlled by a second UHF station on a channel removed by 75 KHz or more. The required technical standards would have to be adhered to. Likewise a 27 MHz CB channel could be used for control, however it would be prudent to consider that unwanted stations may be relayed to the UHF channel when the band opens. Tone operated squelch should be used on the link where legal to do so.
For linking of UHF stations, the use of yagi directional antennas can improve the isolation between the two closely spaced channels.
To be on the safe side of the ACMA Certification process, the OP should look for ways to make the remote base operate without any internal modification to the radios. Specifically the COR circuit may need to be obtained externally by gluing a photodiode to the radio(s) call light or detecting audio amplifier activation from the speaker leads through VOX (crummy) or DC voltage shift (nice if you can). Or you can simply find two older high end radio models that bring out the COR and PTT to an option connector.
Here is a pretty old article on remote base operation. It goes into detail of multiple links and duplex audio which are not necessary in this case. The hardware requirements in the current case are minimal, just two UHF CB radios, two antenna systems, a link interface (passive cross connect or active controller) and a power supply.
http://www.repeater-builder.com/tech-info/pdfs/remote-base-alternative-to-repeaters.pdf