I tried that out and got an error too. I suspect you'll have to book through an agent or over the phone with Qantas. I would imagine that Qantas would waive the phone booking assistance fee as you're legitimately unable to do this booking online.
Qantas multi-city engine is very "temperamental" - I often need to phone and yes the fee is waived (don't hesitate to ask if not offered).
Is it just me or has the multi-city engine gotten even worse in the last few days? I now get 66002 errors on even very basic multi-city trips, predominantly with a Trans Tasman leg. There's no issues with timing or QF flying the routes, or doing any fancy positioning flights, just simple sectors failing to book as a multi-city. I'm talking really simple ones like CBR-AKL -- AKL-BNE -- BNE-CBR. Used to have no problem doing these and much more.
Of course can easily book individual legs, but that's more paperwork, higher credit card fees, and flights on different booking numbers which can be an issue.
Temporary glitch or change of approach?
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I am planning a trip to London, Frankfurt then Amsterdam and then home to Sydney. Each time I get a variety of prices coming up... Definitely seems to be playing up the last few days as I looked at booking it a month or two ago and didn't have these issues. I thiit could be going from Sale to Flexi but a simple refresh seems to solve it...