Sadly, the only insight was that the steep, mountainous central North Island section of line was chosen for the benefits of electrification in those areas, even though twice the amount of freight was being carried between Auckland and Hamilton.

I've requested the book "Rail Electrification in Australia and New Zealand" from the State Library of Queensland. Hoping that the answer will be therein contained. I will let you know anything that I find out.

I've been digging out what I can about this but can't find any definitive explanation as to why only the central section of the North Island Main Trunk line was electrified. But I will keep digging...

Fremantle Line train to Freo - lots of stuff to do there.
Yanchep Line train to Glendalough then bus to Scarborough Beach
Fremantle Line train to Cottlesloe - then walk to the beach to see how the rich live.
Ferry to South Perth from Elizabeth Quay
Kings Park & Northbridge near the city centre