My own experience suggests that after a 15km ebike ride you will probably still want to take a shower. In summer, just the fact that you are moving your legs will get you in a lather, and in winter, there is a high chance of getting wet. My advice is to accept the inevitable and assume that you need to change (plus/minus a shower) at the other end.
Then there are sections of the Waikato River Trails (eMTB although some sections are less technical), Ohakune Old Coach Rd, Fisher Track, 42 Traverse, Bridge to Nowhere, Great Lake Taupo, Motu Trails, Remutaka. I hope to ride and write these up some time.