Phil Goff's aides must be smoking a particularly powerful brand of Kronic if they think a capital gains tax will pull in $4.5 billion for a Labour Government to spend.

It was just February after all when Goff was hosing down suggestions that Labour would embrace a capital gains tax. This week his spin-meisters planted stories that a Goff Government would pull in an extra $4.5 billion a year, all through finally slaughtering the longest-living sacred cow of the Kiwi tax system.

But if Goff throws in compulsory superannuation and says he will bump up the qualifying age for national super to 67 years, he may well get Labour back into the electoral race as a party that is prepared to tackle the big issues.

Sure the Government's coffers will get a rather nifty boost if a capital gains tax is applied to farm sales, business sales, commercial building sales, beach house sales and indeed the residential investment properties sold on by so-called "speculators". Of whom the Labour leader is of course one, given that he owns a "renter" in Wellington.

But it won't have escaped notice that many New Zealand asset classes are struggling to hold value - particularly house prices, which escalated madly during the recent property bubble.

This never seemed to trouble Goff all that much during his time as a senior Cabinet minister in Helen Clark's Government. But it's a fair bet that his newfound resolve to milk the capital gains tax cow is at the behest of ambitious colleagues like the "two Davids" - Messrs Parker and Cunliffe. These are the men who have been in the backrooms developing a new agenda for Labour. Not Goff. Though he will be the beneficiary of their boldness.

What the slow leak of Labour's new tax policy has done is bounce Goff back into the limelight when his lengthy absences from the fray were inviting speculation he was overseas preparing for a post-politics career.

But the full-on retaliation Labour's leakers provoked from John Key suggests he, too, might be susceptible to political Kronic: "Hellfire and damnation will erupt if New Zealand introduced a capital gains tax. The tax avoidance industry would go into overdrive" ... you get the drift.

A bravura performance made rather delicious in that Key obviously felt no irony that he had managed to acquire his own substantial business fortune in jurisdictions that sported the dreaded capital gains tax.

And so it went on and on.

We won't know the details of what Labour is actually proposing until Thursday. But if a broad-brush capital gains tax is imposed for sensible reasons - not an illusory revenue grab - then it is surely time that New Zealand moved down this track. Labour has to present the introduction of a broad-based capital gains tax as a mechanism to ensure a fairer and stronger tax system. One which will result in more investment shifting to the productive sector and ensuring
that the wealthy - who easily avoid the revenue man by rolling up their capital assets - do have to contribute.

One of the delicious ironies in watching Goff grasp for his bold new policy agenda is pondering why he and his colleagues did nothing in Government to avert the property bubble.

Capital gains taxes do not stop bubbles. Nor do they make it easier for struggle street to afford their own homes. Australia and Britain have capital gains taxes - but that did not stop their property bubbles.

The reality is that for much of the past decade the world was "awash with cash". Too much cash chasing too few assets. This is what put the hydrogen into the property bubble, not a bunch of middle-income Kiwis sheltering income from the IRD through setting up loss attributing qualifying companies - LAQCs - to avoid the 39c top personal tax rate that Labour introduced.

The previous Government could have put a dampener on this by wiping the use of LAQCs. Or talking seriously to the Reserve Bank on the need to introduce loan-to-value ratios to stop the "low doc" deals that far too many New Zealanders signed up for to buy houses that they couldn't really afford.

But the Labour Government turned a blind eye - preferring to be the political beneficiary of a so-called economic boom.