Re the railways, the government has actually bought up only the track, while new private sector owners now own and run the operating company. It is probably a little unfair to suggest that the purchase was the (direct) result of widespread voter dissatisfaction – Tranzrail, the previous private sector owner, had almost ended up in liquidation. Without that trigger, this government would have had no great appetite for getting back into ownership of railways. Also, there is at least a case that the lines should always have been separated from the operating business.
As for whether rail should exist in New Zealand, I think that is still an open question (although, like everyone, I was surprised at what the original bidders paid at privatisation in 1993). The issue is not about passenger services, except, maybe commuter lines in Wellington – the population is too sparse for economic inter-city services – but about a limited network for freight needs (mainly bulk dairy and forestry). The very sparseness of the population, and the rugged terrain, also makes good quality roads a challenge to justify/maintain.
As for Air New Zealand, perhaps one can say only two things for the defence. First, late 2001 was the worst time to be relying on new operators to provide longhaul airline services (recession and 9/11) and, sensibly or not, almost any government in the world would have done the same thing at that time. And second, at least so far it looks to have been a good deal financially – Air NZ was sold for more than the government bought it back in 2001, and its market value is now above the latter price. As for the Qantas deal, the curent NZ government has actually been quite supportive of it, but the transaction was blocked by competition authorities on both sides of the Tasman (concerned about extreme market dominance on NZ domestic routes and most (non-auckland) trans-tasman routes.