If you have a look at SYD, the same applies there. At both the F and J Qantas lounges, only 1 guest is permitted, even though the Qantas lounges are the designated lounges for non-Silver members.
Now that I've had a look at some more examples, it appears that where the outstation lounge is operated by another oneworld airline (eg QF or AA), it is Diamond + 1 guest. If the lounge is a CX lounge or a Plaza Premium-type lounge (eg Aspire etc), then it is Diamond + 2 guests.
Good catch on SYD (and MEL). The lounge pass likewise is not accepted here. I wonder, though, if DM can bring 2 guests (and everyone can use lounge pass) if he or she asks for the Aspire lounge invitation. CX only recently decided to not use QF lounges exclusively in SYD, as reported by
https://www.executivetraveller.com/n...-sydney-lounge last month.
On the other hand, I do not see such restrictions at
https://www.cathaypacific.com/cx/en_...ss-lounge.html and
https://www.cathaypacific.com/cx/en_...velazquez.html, so I do not think this is a general oneworld lounge thing.