Hi Vicki good to see you are still doing the animal thing lol
to answer your questions
What kind of frogs would do best in the Far North of nz? Our house can get very warm in summer.
Both green and golden bell, and southern bell will handle the far north caresheet here https://aquariumworld.nz/databases/22-Tropical FWFishallowedinNZdatabase/1850-Green-and-Golden-Bell-Frog
What kind of water to land ratio is ideal for them, and what kind of filters are recommended?
A small bowl will suffice so land as general environment has good humidity, about 30% water if spawning
Also how many frogs are best to keep? Are they social or is 1 happier?
They get on okay if similar sizes but big frog is always capable of eating little frog
A small filter just to keep some movement in the water works well