I haven't been able to find information on what part of their overall network is impacted. Per [1] they use a mixture of leased fibre and owned fibre for connectivity between Australian cities, and per [2] (note: zoom in) many point-to-point microwave links connecting mobile phone tower sites to the fibre footprint. It's potentially as bad as Optus needing to send a technician on a flight to a remote airfield somewhere in rural Australia, then driving 5 hours to a microwave tower to fix a router within the communications hut. Multiply by thousands of routers which are no doubt very much spread out. If Optus are lucky, it's just impacting routers for part of their overall network. For example, only impacting routers within data centres of the major Australian cities where you'd expect Optus to have technicians nearby who could physically access the router with 1-2 hours notice.

[1] https://www.optus.com.au/dafiles/OCA/Wholesale/ProductAndSer...

[2] https://maprad.io/us/search/text/optus?filters=WyJkdl9saWNlb...