New Zealand's economy could miss out on up to $50 billion worth of economic growth because of the Zero Carbon Bill, according to official analysis.

But Climate Minister James Shaw tells the Herald the figure does not worry him and says the cost of doing nothing could be much higher.

A regulatory impact statement (RIS) for the bill also reveals a number of "innovation assumptions" needed for New Zealand to reach the target of net zero greenhouse gas emissions – apart from biogenic methane – by 2050.

This includes 95 per cent of light vehicles and 50 per cent of heavy vehicles, such as trucks and buses, being fully electric in 30 years' time.

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The RIS, written by officials at the Ministry for the Environment, reveals that New Zealand's economy will grow to $522 billion by 2050 if the zero carbon legislation is not adopted and the status quo is maintained.

However, if the legislation does come into force – as it is expected to later this year – the RIS showed New Zealand's GDP in 2050 will be between $472 billion and $476 billion.

That is a difference of $45 billion to $49 billion.

But Shaw said the figure does not represent a cash loss, as the forecast suggests the economy is still growing.

Rather, the number is just a change in the rate of economic growth.

Plus, the figure does not take into account the economic impacts he said climate change would have on New Zealand.

For example, the estimate does not factor in the cost of droughts, floods, storms and rising sea level – all of which would cost the economy billions of dollars, Shaw said.

In fact, he said the numbers should be "taken with a grain of salt".

"If you asked someone in 1935 to run a computer model for the economy in 1965, but you hadn't mentioned things like World War II, you wouldn't be able to say the [modelling] had much accuracy to it."

The Zero Carbon Policy was released by Shaw and Prime Minister Jacinda Arden last week. They have both referred to it as being "ambitious".

It commits to what is called a "split target" – aiming to reduce all greenhouse gas emissions, aside from biogenic methane, to net zero by 2050.

Biogenic methane – the emissions created from livestock such as sheep and cattle – is not completely exempt as the bill commits to reducing it to 10 per cent below the 2017 levels by 2030.

The bill also commits to reducing gross emissions of biogenic methane to between 24 and 47 per cent below the 2017 levels, by 2050.

But for New Zealand to able to reach the 2050 target, the RIS makes a number of assumptions.

Officials assume that 95 per cent of cars and 50 per cent of heavy vehicles would be electric by 2050.

Shaw said he thought that was actually a bit on the conservative side, and said he thought 100 per cent of cars would be electric by 2050.

"Simply because the cost of maintaining a petrol vehicle by 2050 will be so high there would be no point in running [it] and all of the petrol stations will have closed."

The RIS also assumes that New Zealand will have 98 per cent renewable energy from 2035 to 2050, with the other 2 per cent coming from gas.

A methane vaccination programme to reduce livestock emissions would also need to be commonplace and dairy emission would need to be reduced by 15 per cent.

Asked if these assumptions are too ambitious, Shaw said no but added it would be "challenging".

The official analysis also showed one of the options for the Zero Carbon Bill was to have no split gas target at all and for the target of the legislation to be to reduce all greenhouse gases to net zero by 2050.

This would have been more in line with the Labour/Green Party supply and confidence agreement which stated one of the Government's goals was to: "Adopt and make progress towards the goal of a Net Zero Emissions Economy by 2050".

No mention of a split target.

Asked if he was disappointed that the option with a split target was adopted over the one with a blanket zero net greenhouse gases approach, Shaw said: "personally, yes".

"I did think it is the safest option," he said and added that actually, the difference between the split target and targeting all greenhouse gases target was "really marginal".