Parliament's environmental watchdog says the Government should put up a lower methane target in its proposed emissions-reduction bill to get the legislation in place faster and with wider support.

A parliamentary select committee is currently taking submissions around the country on the proposed "Zero Carbon bill".

The legislation sets a goal for greenhouse emissions across the country to be lowered to net zero by 2050, except for biogenic methane – which comes from cows and sheep.

It asks for those methane emissions to come down by 24 to 47 per cent below 2017 levels by 2050 – in what has proved to be one of the most contentious parts of the bill, particularly among farmers who have described it as an "unsubstantiated aspiration".

On Thursday, Parliamentary Commissioner for the Environment Simon Upton told the committee he believed the Government needed to pick a single target, rather than a range.

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"In the interest of achieving agreement and consensus and getting a bill that everyone signs up to, I'd go for the bottom end of the Government's target," he said.

"But that's a political judgement. If you can do better because we get a methane vaccine, or something, then you can amend that."

Upton said the methane target had not been subjected to enough of a "comprehensive analysis" and needed to be put through more modelling to make sure it would be considered beyond political debate.

The figure, for example, had not been analysed for its impact on temperatures.

"That is the environmental outcome we are all trying to contribute to," Upton said.

"Subjecting the target that finally made it into the bill into the kind of comprehensive analysis, I've suggested, could only help to give it credibility.

"If it ends up being relitigated because its consequences were not properly anticipated or understood then the very durability and certainty that the target is supposed to provide to communities and businesses will evaporate."

But asked if the legislation needed to be delayed for changes, Upton insisted the process had already been dragged out for too long.

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"I think you should get the bill passed. Get it passed and then if there are matters which you want the Commission to come back to you on - and be prepared to amend it ... you could do that," he said.

But the majority of Upton's submission on Thursday focused on a call for the Government to urgently consider adding in caps for how much forestry could be used to offset emissions.

"Easy recourse to cheap forest offsets will delay action on tackling gross emissions and mean a much larger remaining emissions problem after 2050," he said.

"A restriction on forest off-setting must be put in place."

Natural carbon sinks were at risk from disease, fire, climate change, and large-scale forestry would also have an impact on the wider environment, Upton said.

Hearings on the Zero Carbon bill continue and the bill is expected to be enacted by the end of the year.