My experience of this was coloured both positively and negatively by my mistaking the Southern end of the Atene track for the beginning of the viewpoint track.
My legs can attest to the fact that this was not the gentle family day out walk. The track shoots straight up the side of the valley going through lowland forest then through into mountain beach. Really pretty. Lots of Tom tits and Kereru to keep you company - Norwegian hunters take note! - I only made it as far as the Taupiri trig as I wasn't prepared expecting as I was the viewpoint. I spent about 30 minutes there looking around. Beautiful views right to the coast but it was getting dark and it was damp and there are sections where there is an invigorating drop off on either side to keep you careful with your foot placement. Going back up next weekend to do the full circuit. It took about 40 minutes of reasonably hard climing to get to Taupiri trig and it goes up higher after that too so I suspect it would be a decent day's exercise. I certainly wouldn't bother trying it as a family day out. I can imagine a few terse worrds being exchanged as a light afternoon stroll turns into a jomp up the side of a rather more trying hill.. That said you would have to try hard to injure yourself and if you take a few breaks, it's a beautiful place. More details after the weekeknd. My ranking reflects that fact that I am a very harsh critic and I haven't completed the full circuit yet. If you aren't doing anything in Wanganui then get out here and do it. Really worthwhile and good exercise too.