Further cuts to spending or increased taxes will be needed after the election to avoid Treasurer Scott Morrison's ambitious company tax cuts sending the budget backwards, according to the independent advice relied upon by the government to justify the key election promise.

The tax cut will deprive Canberra of more than $8 billion a year in revenue while recovering only a fraction of that through the extra growth in jobs and wages predicted.

While the government expects more than half of the shortfall to be recovered from greater economic activity, the modelling reveals that nearly all of it is to be recovered from companies voluntarily deciding to do the right thing through a dramatic improvement in the level of corporate tax compliance, which has been described by some economists as "ambitious".

"The major source of 'self funding' for the corporate tax cuts isn't increased wages or employment, it is the assumption that companies will voluntarily engage in far less profit shifting ... it's bizarre," said Richard Denniss, the chief economist of left-aligned think tank, the Australia Institute.

The government announced in its May budget that it would cut the corporate tax rate from 30 cents in the dollar to 25 cents for all companies by 2026-27, and conceded some days later that this move would cost the budget an estimated $48.2 billion over the decade.

Key modelling behind the proposal undertaken by Independent Economics' Chris Murphy states that the cut would be mostly self funding because, in addition to improved revenue flows from stronger jobs growth and higher wages, fewer companies will attempt to hide profits or send them abroad.

However, in Dr Murphy's detailed report to Treasury, the scale of any new compliance by companies has been set at a high level: "... the reduction in profit shifting from the company tax cut reduces payments abroad by around $4 billion..." he wrote.

Even if that return is achieved, and other economists said this cannot be assumed without further explanation from Treasury, Dr Murphy warned other taxes would need to increase anyway to cover the remaining shortfall of the company tax cut.

These could include an, "increase in personal income tax through bracket creep; an increase in personal income tax through an income levy; and, a reduction in spending through government efficiency savings".

Neither Prime Minister Malcolm Turnbull nor Mr Morrison have stated definitively where the money to pay for the company tax cuts would come from.

While declining requests for detailed costings beyond the first four years of his decade-long glide path, Mr Morrison told the National Press Club on Friday that the company tax was affordable.

"If you look at the medium-term projection you will see that the underlying cash balance over an 11-year period keeps us in a positive cash balance all the way out which includes the corporate tax cut in the estimates," the Treasurer said.

But Dr Denniss said the modelling "took for granted that a cut in the corporate tax rate will lead to 20 years, or more, of bigger budget deficits than would otherwise be the case".

"While the fact that a policy might increase the size of the budget deficit for decades should not be used to dismiss the benefits of such a policy, it is interesting to note that the net benefits of the proposed corporate tax cuts have not been compared by Treasury to the net benefits of other policy proposals with long gestation periods," he said.

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