Some features it supports:

• measuring distance and drawing tracks
• generating elevation profiles (using the LINZ elevation API)
• automatically caches areas you've viewed for offline use
• viewing mountains and routes from https://climbnz.org.nz/mountains
• viewing doc huts
• live weather (scraped from Metservice)

The source code is available here: https://github.com/fallaciousreasoning/topo