Airlines like QF have charts which designate certain airfields as "Main", "Alternate", "Adequate", "Emergency" etc, and are specific to certain aircraft or groups of aircraft.
I believe Learmonth is classed as an "Alternate" for a QF A330, whereas Port Hedland and Broome are "Adequate". Not sure what the exact criteria are ("Adequate" for ETOPS?), but am having an educated guess that an "Alternate" is a higher preference than an "Adequate" or an "Emergency".
No doubt the crew balanced the factors at hand, and of course readers at home enjoying a peaceful afternoon may have wide and varied opinions based mostly on lack of information...
