Think of a figure, double it, add your age and subtract the number of eels in Oakley Creek: that, it seems, is as good a guess as any for the price of completing the Waterview Connection.

I'm not surprised Transport Minister Steven Joyce and the NZ Transport Agency waited until the eve of the Christmas exodus to sneak out the highly embarrassing news that a tunnel was, after all, the most cost-effective and environmentally sensitive way to join State Highway 20 up to the Northwestern Motorway at Waterview.

Mr Joyce called this backflip, a "fine-tuning of the alignment", while Transport Agency chairman Brian Roche's press release was even more buttoned-up, referring to "design improvements". The only reference to cost was by Mr Roche, who said the new tunnel "can be completed within the original project budget and is the most cost-effective option for constructing this section while also responding well to community concerns".

In other words, at Christmas 2009, a three-by-three-lane tunnel could be built for around $1.15 billion. Yet just 10 months before, Mr Joyce signed the death warrant for the defeated Labour Government's similar tunnel project, saying costs had blown out from $1.89 billion to "in excess of $3 billion". On closer examination, his bloated $3 billion included all sorts of add-ons the Labour-backed plan did not.

Even with the politicking put to one side, what is alarming is the huge variation in estimates provided by the bureaucracy for similar outcomes. This time last year, the incoming National Government received a briefing from the Treasury and the Ministry of Transport on the business case for the Waterview Connection.

The Transport Agency's preference, which Labour had endorsed, was a bored tunnel, with two lanes in each direction. The estimated cost in 2008 was $1.752 billion. The three-lane option would be $2.039 billion. Various cut-and-cover options were listed, but all were considered less acceptable, socially and environmentally.

Mr Joyce found them all too expensive and demanded a rethink. The agency came up with a hybrid solution involving a tunnel, a surface stretch, and a section of cut and cover. It was a compromise which made no one happy except the Government, who couldn't see past the bargain $1.15 billion price tag.

What's still to be explained is how, in December, the experts suddenly decided they could come up with a three-by-three tunnel solution costing not much more than half their original tunnel estimates. In September, the Transport Agency board voted that the bitsy compromise scheme "was the best option to complete the Western Ring Route within available funding". Mr Roche said it was "affordable" and "provides the best balance between the need to complete the Western Ring Route with the impacts on surrounding communities". But within three months, this "best option" was scrapped and a tunnel was back in favour.

True, the original tunnel was to have been 3.2 km long, compared with the 2.4km of the new tunnel. Also, instead of using an expensive boring machine, the roading authority plans to fall back on less sophisticated tunnelling techniques. We are told that between the two estimates, the road builders got "a better understanding of the local geology". Fair enough, but as I understand it, both tunnelling projects are based on the same premise, that it's easier to go underneath the lava flow running north into the upper Waitemata Harbour, than to try to cut through the hard-surface volcanic rock.

You have to ask why this cheaper tunnelling option was not considered earlier. When, for example, locals were up in arms about their neighbourhood being cut in two. And especially a year ago when Mr Joyce was signalling his and his Government's intention to scrap the full tunnel plan in favour of a cheaper, and more environmentally and socially unfriendly, option.

There's no doubt the reversion to a single-tunnel solution, albeit shorter than the original, is to be applauded. That it took the transport boffins so long to come up with an affordable way through is not.