Lessons for Auckland?
The Northern Airport Line will be Light Rail while I am pushing the Manukau to Airport section of the Southern Airport Line to be Light Rail (Manukau to Botany will be bus).
The first is about capacity to meet demand (The Southern Airport Line intersects Manukau Bus Station and Puhihui Train Station that have heavily patronised transit lines running through them) and the second is about ambience especially in a major Metropolitan Centre like Manukau.
Manukau or rather Transform Manukau is 600 hectares in size compared to the main City Centre being around 385ha (this does NOT include Port of Auckland but does include Wynyard Quarter). Manukau is also in the middle of a large regeneration program that includes intensification and upgrades of public spaces. Manukau is also forecast to house 20,000 new residents (I say it can hold double) over the next decade or so.
Simply put given its large size busses are not going to quite cut it moving people around Manukau nor to and from it to another major employment area like the Airport. Ambience is also in play and like the City Centre having a high amount busses inside Manukau doesn’t exactly work either.
Running higher capacity trams along the Northern Airport Line through the Airport and down to Manukau via the Southern Airport Line would be Auckland’s example of my Airport to City Centre LRT via William Boulevard Line coupled with the Airport to Laytonville Metro Line.
A LRT Line doing a circuit of Manukau and Manukau City Centre linking up the main commercial area, main residential area to the south, the events centre to the south-east and AUT to the north-east would be an example of my City Boulevard Lines.

FULL POST via Trans City vs Local District Transit. #CitiesSkylines Lessons for Auckland
But heavy rail (ALSO) to the airport could be direct connections to both Hamilton & Tauranga ? as well as all present rail in Auckland ?
CRD
Not without great difficulty at Puhinui. The Southern Airport Line either as BRT or LRT continues on to Manukau then Botany and Howick widening its catchment. HR to the Airport wont do that and a transfer at Puhinui would suffice.
Would be the same if we ran HR from Otahuhu to the Airport. Hamilton and Tauranga passengers would still need to transfer at Otahuhu.