Originally Posted by
Globaliser
I think that the problem here may be that ba.com is for some reason not seeing the BA codes between HKG and MEL, and that this part of the website will only book flights on BA codes. It may be the way it's asking for availability and/or because of rules about when the HKG-MEL sector can be sold as a BA code. ...
You might want to have a play with the ITA Software site to see whether the problem is that fare rules do not permit HKG-MEL to be booked as a BA code if you have a stopover in Hong Kong.
Having had the time to play a bit, it looks like the problem may be this: The fare rules for BA business class fares to SYD and MEL appear to provide that you cannot have a stopover in Hong Kong if you continue to Australia using the same airline's code. So you can fly LHR-HKG-MEL on BA code followed by BA code providing you connect at HKG and go on to MEL straight away. That should therefore be bookable on ba.com.
However, if you have a stopover, you can't continue HKG-MEL using a BA code. You can still use the fare if you book a CX code or a QF code on HKG-MEL, and the ticket can still be priced using BA fares. But ba.com won't book those for you using the multi-city function because there's a sector that doesn't have a BA code, and that's why you're seeing the error message that you're getting.