Actually, it was 50,000 watts back in the day - certainly in the 1990s (I remember the figure because I visited it in 1994 and the tech at the site told me).
Personally I've received the 567AM signal as far south as Dunedin, and as far north as Auckland (both with a Sony Walkman!), so I don't imagine covering all of Taranaki would have been much of a stretch for it.
The old tower (220m tall - not far off the Sky Tower's original height) was demolished in 2016 due to age and spare parts becoming scarce, and all transmission services moved to a newer, existing transmitter at the same location.