Hi 400volt, welcome to ECN.
Robin KMP 4120DL Earth loop impedance tester with D-Lok, claim not to trip most RCD's.
I had one on loan from my local wholesaler recently when my multifunction tester was away for repair. I thought it was the business as it didn't trip any of the 30mA RCD protected circuits I used it on.
I don't know if they are available down under. You could visit their web site www.robinelectronics.co.uk
& perhaps contact them to see if the tester meets your needs, & if they have distributors in Australia.
I believe Megger make a loop tester that doesn't trip RCD's, part no. LT7. I don't have a web address for them.
Loop impedance testers prove that you have a low impedance path to earth, to ensure that the circuit protective device (mcb/fuse) will operate within the times required by BS 7671 (or local equivalent), when a line-to-earth fault of negligible impedance exists at the remote end of the circuit. In the UK this is known as the shock protection constraints.
It also checks that earthing facility provided by the supplier is of low impedance.
In the UK, the external fault loop impedance of the suppliers earthing facility must not exceed:
0.35 Ohms for TNC-S (PME / MEN) system,
0.8 Ohms for TNS system.
Knowledge of the earth fault loop impedance value (Zs) is also required where cables with reduced protective (earth) conductors are used, as in UK twin & earth cables.
This is to ensure that the thermal constraints of the protective conductor will not be exceeded, ie. The reduced earth wire is large enough to cope with the fault current without damage.
Finally, here in the UK BS 7671 test procedures & certificates require values of earth loop impedance (Zs) to be measured & recorded on the test certificate.