Business New Zealand chief executive and Tax Working Group member Kirk Hope says the proposed capital gains tax could cost the economy billions of dollars in compliance costs, administration and lost productivity gains.

Business NZ yesterday released a report that estimates the "economic drag" over the first five years of the proposed CGT regime at between $2.75 billion and $6.81b.

Hope — one of three Tax Working Group members who did not back the capital gains tax proposal — says it is the first serious estimate of costs associated with the proposal.

There was reference in the TWG report to potential costs but there weren't numbers produced, he said.

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Business NZ had made assumptions based on CGT regimes in places like Australia, which would hopefully get some debate started, Hope said.

They had the work independently assessed by economists.

"We're not trying be bigger than Ben-Hur here," he said. "We wanted to produce as realistic a picture of what the costs for business and the economy would be as a result of simply complying."

A range of assumptions and estimates had been made which Hope said he believed were conservative.

"If other groups have different assumptions and estimates that's great but we want to see some debate about the costs side," he said.

One of the biggest areas of potential cost was the "deadweight loss"on the economy.

That was, essentially, the opportunity cost, or value of lost output, when a tax regime prompted people to switch from higher valued to lower valued economic activities.

That was one of the biggest problems with the capital gains tax proposal in its current form, he said.

The kind of young, innovative companies that drive economic productivity and create jobs tend to retain and grow capital rather than paying dividends meaning they would be hit hardest by a capital gains tax.

The deadweight loss was estimated to be between $1.5b and $4.2b. The estimated also included compliance costs of $1.6b over the first five years and administration costs of $210m over the same period.

Compliance includes Valuation Day costs, included in TWG calculations, which the Business NZ report estimates to be worth $800m.

"Based on evidence from Australia, CGT compliance costs could be as high as 16 per cent of tax revenue, we have assumed 10 per cent on the basis that NZ's CGT is less complex," Hope said.

Business NZ had not included valuation costs for private businesses as they were not included in TWG calculations, he said.

They could potentially take the real cost of the CGT even higher.

Administration included IRD's collection and enforcement cost.

Business NZ also argued that if proposal was not revenue neutral — with no counter balancing tax breaks — the total economic drag over the first five years could be much higher, potentially as much as $14.8b over five years.

The estimates were given to Finance Minister Grant Robertson's office. A spokesperson for the Minister declined to comment saying the Government was assessing the Tax Working Group proposal and would make further comment later in this month as previously indicated.

The Government has said it is not bound by any of the recommendations of the tax working group.

It has said any tax regime changes — including a capital gains tax — would be put before voters as policy in the 2020 election before being introduced.

It has also ruled out any capital tax being imposed on the family home.