https://www.youtube.com/watch?v=OmtSI3N57kM Auckland, like Brisbane, Sydney, Melbourne, Perth, Seattle, Edmonton, Toronto & Montreal... are just some of the cities that get to have longer train than provincial Vancouver.
https://en.wikipedia.org/wiki/City_Rail_Link
"Although the trains are capable of having extra cars added in groups of three, the CRL station platforms, as originally specified, would not be long enough to accommodate nine-car trains. The proposed new capacity is 54,000 passengers per hour with the station platforms to be made longer so they can take the longer trains..." https://en.wikipedia.org/wiki/City_Rail_Link#Capacity_forecast_forces_platform_enlargements
9 car trains are possible for Auckland, because there isn't anything like a Vancouver Mind Virus (VMV) to thwart it.
https://www.cityraillink.co.nz/meet-your-stations
https://en.wikipedia.org/wiki/Waitemat%C4%81_railway_station#Capacity Proper big city infrastructure planning and development, unlike backwards Vancouver.
https://en.wikipedia.org/wiki/Waitemat%C4%81_railway_station#Electrification
In contrast is the joke that is the two car Canada_Line. There was a deliberate decision, (apparently to save money) to not have a future station level clearance of at lest 100 m, if not even 160 m. So instead of planning for at least 5 car trains, there is only enough space for a 2.5 car joke of a train, some day.
https://en.wikipedia.org/wiki/Canada_Line#Stations "The five busiest stations have platforms 50 metres (160 ft) long, while the rest of the stations have 40-metre (130 ft) platforms that can be easily extended to 50 metres."
Being that this is a line which could eventually link YVR to the Tsawwassen Ferry Terminal & eventually Waterfront station with Park Royal and even the Horseshoe Bay Ferry Terminal, the BC Mind Virus (BCMV) got in the way.
https://en.wikipedia.org/wiki/Canada_Line#Technology "The capacity of the trains is estimated at 334 people per pair of cars (comfortably) or 400 people at crush load."
Since each car is 20 m and the stations are designed to a maximum of 50 m, every effort should be made to try to eventually have 60 m platforms. Unfortunately, it might be difficult enough just to try to gradually have 55 m stations. Still, the initial goal should be to try to have a 3 car train of 60 m.
Then with Selective+Door+Operation, a 3 car train could become a 5 car train. Its unfortunate that there is such bad transportation planning in Greater Vancouver. Compared to backwards Vancouver, its great that nothing like a VMV has prevented Auckland from eventually having 9 car trains. The Montreal Metro and the Stockholm Subway can also have 9 car trains. Unlike Vancouver, several cities have no problem thinking big.
Auckland is so far away from the backwards mentality of Vancouver. Thus, Auckland is allowed to have longer trains than provincial Vancouver and taller buildings and even a wider harbor bridge. Of course the same can be said about Sydney and several other cities.