Here in Auckland, it's hard to picture our singing, dancing, vision-happy mayor as the antipodean Frankenstein's monster. But down in the Beehive, Government ministers must be starting to wonder what they've created.

Sure, they wanted a new united city with a unifying cheerleader at the helm, and this wish has come true.

Len Brown even talks the same jargon the Government used to sell the Super City reforms, enthusing about the "transformational change" needed to lead the region and the country out of decades of economic malaise.

The only problem for Prime Minister John Key and his team is that their creation has come with an annoying flaw - it has emerged, fully grown, with a mind of its own.

And high on Mr Brown's list of things to do is to honour his campaign pledge to build an underground inner-city rail loop, a project very low on the Government's priority list.

Not only is Mr Brown staking his reputation on getting the tunnel built, he's lined up support in the council.

Transport chairman Mike Lee was always going to back it. As chairman of the defunct regional council, he's been fighting for a modern commuter rail system for years.

More alarming for the Government is that former National minister and Auckland Mayor Christine Fletcher is a fan and says as leader of the centre-right opposition on the Auckland Council that her team will be lobbying their co-religionists in the National caucus to get it built.

After the release, on Wednesday, of a highly enthusiastic $5 million business-case report, Transport Minister Steven Joyce poured cold water on the project, noting the estimated $1.9 billion cost did not include the $340 million bill for new trains and rail tracks. He also tried to play down the stunning, "wider economic benefits" claims of $3.50 for each dollar invested.

But what he can't hide from is that even using more conservative calculations, the rail tunnel's benefit-cost ratio matches that of the controversial Puhoi to Wellsford "holiday highway" which the Government plucked from nowhere and anointed as a "road of national significance".

What's also embarrassing for the Government is that the report, commissioned by state-owned KiwiRail and the now-deceased Auckland Regional Transport Authority, takes borrows all the arguments the Government used to justify creation of a Super City, and insists the CBD tunnel is vital to finishing that job.

Indeed, the CBD tunnel "is not just an essential element in Auckland's transformation into a globally competitive urban centre, it is perhaps the most critical." (The report is online at

The report says to become "a strong international city" Auckland needs increased "agglomeration" at the centre, to ensure an effective density of highly productive industries, busily exchanging staff and ideas as they merrily prosper away.

It cites OECD reports blaming New Zealand's low standard of living on low labour productivity, and notes that worldwide, growth tends to occur in metro-regions which have higher GDP per capita than the national average, and also faster growth rates. It quotes a Motu Research report that the Auckland CBD "value added per worker" is 63 per cent higher than the rest of the region, and more than double the New Zealand average.

In other words, the more businesses you can pack into the CBD, the faster national living standards will rise.

And to attract the young and intelligent workers from here and abroad needed to achieve this breakthrough, Auckland needs the rapid and efficient public transport only achievable through a rail-based solution that makes the city liveable. It's the same message the Government and assorted economic gurus have been preaching for eons. But until now, the CBD tunnel element was missing.

Mr Brown and Mr Lee were already converts to this cause, but the report gives them with independent reinforcement the Government demanded. Mr Brown certainly sees it as the signal to steam ahead. The report talks of a 2021 deadline. Mr Brown says he wants it completed in 2017. He has his councillors at his side.

The Government must be thinking the old fragmented days of Auckland local politics weren't so bad after all.