I've got some disappointing news.

The tax changes being talked as budget tax cuts are not actually budget measures. You won't find them in next week's budget bills.

I am also the bearer of even less-pleasing news, which I will get to in a moment. For full-time earners considering putting in an extra hours, the tax changes will increase rather than decrease the tax they will pay on that extra income.

Treasurer Wayne Swan introduced a Bill to give effect to the proposed tax changes the Tax Laws Amendment (Personal Income Tax Reduction) Bill 2008 during the first week of the new Parliament.

A Senate committee has examined the Bill, and given it a tick.

But the fact that the tax Bill is not a budget measures hasn't stopped all of manner of people from talking about it as if it is, from the Treasurer down.

Here's Wayne Swan on Friday when asked at a press conference if working families would be happy on budget night:

"What we have said consistently is that we will deliver the tax cuts because they will be directed primarily at working families, and child-care assistance and the education tax rebate, because they've been hit with price rises. They're doing it really tough and they're our priority in the budget."

On the Insiders on Sunday, Barrie Cassidy asked Swan whether he was still committed to delivering the "tax cuts in the budget".

Rather than let him know that they weren't in the budget, the Treasurer replied that he made "no apology for delivering the tax cuts that we promised".

He even prepared a table for the Sunday Sun Herald that demonstrated that "there is probably no group that are going to be bigger beneficiaries in this budget than second income earners with kids".

Taking into account the tax cuts and the increase in the child-care cash rebate (which will be a budget measure) it demonstrated that mothers needing child care would be a good deal more rewarded for venturing into the workforce.

Wayne Swan has been trying to give the impression that everyone will be.

He told Laurie Oakes in March that the tax cuts would "provide some incentive to people out there who will work additional hours".

But that's not the way his Treasury sees it not for everyone.

Documents issued to the West Australian under the Freedom of Information Act and reported in The Canberra Times last week make it clear that the Treasury believes that a greater number of existing workers will cut back their hours as a result of the change than will increase them.

The total number of hours put in by existing workers will increase only because the people who wind back their hours will do so by less than the people who boost them.

The Treasury's finding that 32,895 Australian workers will decide to cut their hours is striking because it isn't what is meant to happen when marginal tax rates are cut.

The explanation lies in a surprising and little-publicised fact about Kevin Rudd and Wayne Swan's proposed tax change. It will present ordinary full-time earners with a higher effective marginal tax rate.

Two changes will come into force from the start of July (and quite a few more in subsequent years).

One will extend upwards the income range over which people pay only 15 per cent on each extra dollar that they earn and extend upwards the income range over which people pay 30 per cent.

The other will increase the size of the so-called Low Income Tax Offset.

At the moment it is worth $750 and paid in full to anyone who earns less than $30,000.

After July it will be worth $1200 and will continue to be paid in full to anyone who earns less than that amount.

As is the case now, anyone who earns more than $30,000 will lose the payment at the rate of 4c for each extra dollar they earn.

That means that, as is also the case now, the actual loss faced by Australians just above the 30 per cent tax threshold who put in an extra hour of work will be 30 per cent of the extra income plus the 4 per cent they will lose as their offset payment is cut.

The difference will be that after July many more people will be in that situation. The increase in the size of the offset will push the 4 per cent penalty on to more taxpayers.

Here's how it work from July this year. Australians at present earning between $48,750 and $60,000 and losing 30c out of each extra dollar earned will find themselves instead losing 34c. Among the group facing that effective marginal tax increase will be average full-time workers on the current average ordinary time wage of $56,235.

From July next year the upper limit on the effective 34 per cent impost will increase to $63,750, and from July the following year to $67,500.

That means that even as their wages increase, average full-time earners can expect to continue to face an effective marginal rate of 34 per cent rather than the 30 per cent rate they face at present.

These calculations exclude the Medicare levy, which is on top of whatever tax is paid and they apply only to the effective rate applied to the extra hours worked.

Like all of us, every average earner will pay less tax in total as a result of the tax cuts.

The effective marginal tax increase is an unintended consequence of a well-intended change. The boost to the offset will encourage an extra 28,568 Australians at present out of work to give it a go. The Treasury's projections say so.

The decision to continue to withhold the offset from high-income earners is similarly well intentioned. It saves the Government a lot of money.

But withdrawing it adds to effective marginal tax rates. It has to.

None of this necessarily means that the tax cuts are a bad idea just that their effects are complex.

And in any event these particular changes were designed by the Coalition and copied by Labor during the election campaign.

On budget night we will find out what Labor can do with all by itself.

Peter Martin is Economics Editor at The Canberra Times.

peter.martin@canberratimes.com.au