Ho, ho, ho. What further gifts will Father "Matariki" shower upon us Aucklanders in his much heralded address to the Chamber of Commerce on new infrastructure projects today?

Prime Minister John Key has already made all Mayor Len Brown's Christmases come at once this week, with his endorsement of the City Rail Link.

Purely coincidentally, I'm sure, he's also ensured his best buddies at the casino get not just a brand new national convention centre and lots of new gambling machines to go with it, but a fancy underground train station at their door.

Now, like excited kids, we're trying to guess what extra goodies he brings. Rerouting the underground line past the university? A tunnel under the harbour? A super highway through the eastern suburbs?

At the risk of being labelled a party pooper, I suggest not cracking open the bubbly quite yet. For 90 years, central government politicians have been promising us an underground line to Morningside.

I vaguely remember the excitement surrounding the grand opening of the Auckland Savings Bank's new "skyscraper" headquarters at the corner of Queen and Wellesley Sts in 1969. A highlight was the way it had been constructed to accommodate the new underground train service coming up Queen St soon.

"We have designed the building so that the escalators and stairs from Queen St to the tube station will all be on the bank's land," said architect Geoffry Rix Trott.

In October 1923, Railways Minister Gordon Coates talked of a $1.2 million tube line linking a new central station with Morningside, the end stage of the Tamaki-Westfield rail diversion. In 1937, 1949 and the early 1970s, governments of both the left and right revived and then dropped the idea. Mr Key is the latest in a long line.

But he doesn't support the mayor's 2015 start date. He wants a delay until 2020. By then, two general elections will have long gone, as presumably will Sir John, away from the fray as ambassador to the court of King Charles in London.

Unlike his predecessor Helen Clark, Mr Key could never be accused of being a details man. On RadioLive for instance, he said that an on-going worry about improving the rail service was "substitution" of "people getting off buses and on to trains and I think you will get a bit of that but what's also true is that our roads are becoming more congested ..." All a tad embarrassing.

If his officials had properly briefed him, he would have appreciated that people decanting off buses on to trains as their new mode of choice was not a "concern", it was a central aim of Auckland's new integrated transport network - one based on feeder buses delivering commuters to a central spine of railways and busways, to be whisked to and from work.

Also, leaping to the support of the $1.7 billion "holiday highway" north, Mr Key proposed that "my guess is it [the highway] has a higher benefit-cost ratio than the CBD tunnel". Woops. Now admittedly, cost-benefit reports are like theatre reviews, you seek out - or pay for - the opinions of experts you tend to agree with. In the case of the holiday highway, a year ago a Government-commissioned report showed an appalling cost-benefit ratio of only 0.4, meaning a return of 40c for every dollar invested. It was quickly buried.

As for the CBD tunnel, in 2011 Auckland Council claimed a stunning $3.50 cost benefit for every $1 invested.

The Government of course rejected this and commissioned Sinclair Knight Merz to do another. Last November, it came up with a much lower figure of between 78c and $1.05 for every dollar. Though far smaller than the council's $3.50, even the lowest "benefit" was nearly double the holiday highway's 40c.

Mr Key is also very hazy on how the project will be funded, except to say the $2.86 billion cost will be split roughly equally between taxpayers and Auckland ratepayers. He said it would not come out of the national land transport fund because if that happened, "if you live in Invercargill you'll effectively be paying for Auckland's rail loop".

This forgets that for decades, Aucklanders have been disproportionately subsidising the cost of underused highways criss-crossing remote National Party electorates. Even a third of the Government's proposed half-share of costs will come out of Aucklanders' pockets.

So, the green light is a start. But let's not get carried away.