A fantastic 2 or 3-day tramp (hike) for the reasonably fit. We've just done this, mid-Winter, as a fairly out-of-shape 54-yr-old with an 11-yr-old novice (though very fit!) tramper, and had an amazing time. We did it "in reverse", which seems to be what most people do: start at Whangaparapara Rd, along Tramline Track and Forest Road to Kaiaraara Hut (day 1); then up 627m Hirakimata (Mt Hobson) and down to Mt Heale Hut (day 2); then out via Kaitoke hot pools (day 3).
Some key points that you won't find on the Department of Conservation website (if I'm allowed to mention their name by TripAdvisor), which the experienced won't need, but those unfamiliar with NZ tramping/this track might:
1. You probably don't need a map, because it's all so well signposted, but I would *always* take one, not least so you can see where you are and also what you're looking at: you can buy them in NZ -- it's hard to find out where, but unfortunately TripAdvisor won't let me share that info. (the map you want is in the series "Topo50", called "Claris"). (You can't rely on having signal to look at online maps, of course.) Also make sure to take water purification tablets with you: you're better to boil the water, but if you can't, these are sadly necessary -- Giardia is everywhere, it seems. And hand sanitiser is a good idea, both for the huts and as you'll be using the handrails on the stairs up and down Hirakimata. (Other commonsense things, like an emergency reflective blanket, toilet paper, etc. are listed on the DOC web pages.)
2. Parts of the track are incredibly well maintained (almost at "walk in the park" level), especially Kaiaraara Hut to Hirakimata (Mt Hobson) and Kaitoke Hot Springs to the road. For these parts, you could do the walk in cross trainers at any time of the year, even in the wet. Bear in mind, however, that there are *a lot* of stairs (especially up and down Hirakimata -- from the Upper Dam on the Kaiaraara Stream to the saddle between Hirakimata & Mt Heale), so it requires a good deal of stamina -- not for the unfit. The steepest stairs do mostly have handrails, which will save your knees quite a bit! The rest of the track is a little more weather-dependent: there are quite a few slippery bits coming down the Peach Tree Track from Mt Heale Hut, and one (small) stream crossing. This part of the track is also more exposed: I wouldn't choose to do it in heavy rain/strong wind if I could avoid it. Trainers are still okay in dry weather; otherwise, hiking shoes will be more likely to save your dignity. The same goes for some bits of the Tramline Track and Forest Road -- a metalled (stony) road with exposed clay in parts. Overall though, this felt like a "Great Walks" track in terms of the level of maintenance/ease of the paths and the excellent signposting. The one thing I really would take is a walking pole or two!
3. The time estimates on the DOC pages either don't take account of breaks or are drawn up for fit 20-50-year olds. We added an hour or so to each leg of the journey with detours and breaks for snacks, lunch, etc. In Winter in particular, I would advise allowing at least an extra hour in your calculations -- and bear in mind that it gets dark in the forest far earlier than it does in the open.
4. Both huts (which you need to book in advance, esp. in Summer) are well equipped with pots and pans, and both have gas burners -- you don't need to bring a portable stove or pots. You *will* need a torch/torches, as electric light is very limited -- head torch is best. Each hut has two sleeping cabins off the main kitchen/dining room, with an upper and lower platform and 5-7 vinyl-covered mattresses on each. As ever, if you want choice of "beds", arrive early. (We didn't need to worry, as mid-Winter during the week there was very low demand.)
4. I see other people complain about the Forest Road as not a "proper" hike. We liked it because of all the bits off to the side to look at: the Aotea Falls (with a nice pool for a quick dip); Maungapiko, with its views of the Hogs Back; the magnificent stand of large-ish kauris (see photo); and the clear view in the distance of the route down from Hirakimata to Mt Heale Hut (we could even see the hut).
5. For anyone taking kids: you might want to forewarn them that the toilets are all long drops... On the plus side (and again, very unusual for NZ tramping tracks), there are toilets (with toilet paper!) at various points not connected to huts: e.g. near the hot springs, and at the intersection of Kaiaraara Track and Coopers Castle Track.
6. If you don't want to do overnight stays, can move fairly fast, and can manage more than 4-5 hours comfortably, there are various alternative routes that take you from road to road, and still over Hirakimata -- check the Topo map that is freely available online for routes: https://www.topomap.co.nz/NZTopoMap?v=2&ll=-36.212889,175.401964&z=14