Originally Posted by number_6

The IATA rule is resolution 302 which came into effect April 1, 2011. The "Most Significant Carrier" applies, which in the OP's example is CX. If OP had flown AY HEL-HKG then CX HKG-AKL then the AY allowance would apply in both directions for all sectors. If the routing had been BA WAW-LHR QF LHR-HKG CX HKG-AKL then the QF allowance would apply, both directions. The rule is complex, also different if the US is involved (as the US DOT has rules that over-ride parts of the IATA resolution). Note that this affects both baggage allowance and fees for excess baggage -- the latter can be far more significant on some routes.