I was advised last week that Rex will take over the incarriage of Singapore Air flights to SIN and beyond that they have been until now heavily subsidising on the rat.
Hence your DPO LHR all up fare will use ZL to get the punter to the gateway, as will be the case out of regional NSW and VIC.
This is not massive yield, as SPA deals are usually at a very good rate, but has to make things a little more secure for Rex, and much cheaper for SQ.
Best all
EWL