The country's top Treasury official has had to correct her own speech after understating a key figure by about $4,000, and the Coalition wants to know how the department running the numbers behind Labor's budget could be that far off.

Treasury Secretary Jenny Wilkinson told Australian Business Economists last Thursday that median income earners benefit from capital gains and negative gearing concessions by around $5,700 over their lifetimes. The actual figure, according to Treasury's own modelling, is about $10,000. Roughly double.

A footnote and amendment have since gone up on the Treasury website. The department blames a transcription error.

Shadow Finance Minister Claire Chandler says the Coalition will have "serious questions to ask" of Ms Wilkinson when she fronts Senate Estimates on Thursday.

"Labor do not want their toxic taxes interrogated because they know they are full of holes," Senator Chandler told SkyNews.com.au.

"If the Treasury Secretary isn't across Treasury's own modelling, then who is?"

The speech and its figures sit at the centre of how the government is selling its capital gains and trust taxation reforms, which form part of the broader budget package winding back tax concessions that Treasury says disproportionately benefit higher income earners.

Treasury's analysis argues the 50 per cent capital gains tax discount fails to adjust fairly for inflation and has, in its words, "effectively compensated investors." The government says the reforms will move the system toward a "more neutral treatment" across the population.

The modelling claims that if the reforms had been in place since 2000, the lifetime tax benefits for the top 1 per cent of earners would have dropped from $700,000 to around $300,000, and the top 10 per cent of lifetime earners would have paid 60 per cent of the new tax revenue.

Senator Chandler says the modelling "looks backwards" when Australians want to know what the impact will be going forward.

"This policy is an assault on aspiration, including younger Australians trying to get ahead. There is a better way. The Coalition will axe Labor's toxic taxes and deliver bigger automatic tax cuts to help with the cost of living."

Treasurer Jim Chalmers defended the analysis on Nine's Today Show, telling viewers the broader picture still held.

"The difference between $5,000 and $10,000 for the average worker versus $700,000 for the top 1 per cent, I think the point still stands," he said.

"The point that Secretary Wilkinson was making is that people who are already doing very well are the biggest beneficiaries by a long way on the current arrangements."

What these concessions actually are

Two tax rules sit underneath the whole argument, and most of the people affected by them have never had either explained plainly.

Negative gearing lets an investor deduct the shortfall on a rental property, where the rent does not cover the mortgage interest and costs, against the tax owed on their wage income. The capital gains tax discount then halves the tax payable on the profit when that asset is sold, provided it has been held at least 12 months. Used together, the losses come off your income now and the eventual gain is taxed at half rate later.

That is why the figure Jenny Wilkinson got wrong matters more than a slip in a speech. It is the government's own estimate of what an ordinary earner takes out of the arrangements it wants to wind back, and it was out by roughly half in the direction that made those concessions look less valuable to ordinary people than Treasury's own modelling says they are.

It also does not sit on its own. Labor has separately moved on superannuation and on self managed funds borrowing to buy property. For anyone with an investment property or a self managed fund, the practical question is not the $4,000 error. It is how much of the current arrangement survives the budget package.

The Senate Estimates hearing is listed for Thursday.