I recently figured out how to find Mt Taranaki on the horizon, and it's quite easy. The mountain is only sometimes visible — best chances are a fine day with clear air and snow on the mountain.
Mount Taranaki on 22 November 2017. Zoom lens and circular polarising filter helped in this shot.
To spot it, stand directly facing the horizon, with your back to the Tararuas. Imagine the spot you can see is at 12 on an old-fashioned clock face.
If you point to Kapiti Island on your left you'll be pointing at roughly 10 o'clock on that clock face. If you can see the South Island to the right of Kapiti that would be around 11 o'clock.
Now look at one o'clock to your right. That's where Taranaki might be seen.
If you're lucky you may also see Mt Ruapehu at about 3 o'clock — it's just left of the end of the dunes north of the river. I've seen Ruapehu much more often than Taranaki.