For instance, VA only flies five days a week from MEL, five (not all the same) from SYD and four days a week from BNE to LAX, so, given its competitors where they offer this route fly at least daily, VA analysts may decide to offer a MEL - SYD - LAX - MEL return Y fare at the same price as SYD - LAX return. In that case, putting one on a domestic flight from MEL to SYD is "costing" the airline, but it does it because it works out it can't otherwise sell all the seats on its B773ER ex (or to) Sydney, and so given it perceives it's making an overall profit, it's prepared to 'common rate' the fare from MEL.
VA doesn't have the available aircraft, but flying less than daily on major routes like to LAX is suboptimal and puts it an immediate competitive disadvantage. I've used it one way to LAX but returned on another airline as the VA schedule did not operate on the night I was wanting to depart from the USA.