He said National Party estimates on the financial implications might be a bit light because, for instance, "they took a mid-point view on a number of figures".
"For example, a 2018 Landcare report calculated that a charge on methane for a sheep and beef farm could be as high as 123 per cent of their net profits - a nationwide cost in the order of $2 billion and a cost per sheep and beef farm of about $120,000.
"In our view the environmental taxes that have been mooted will be even worse than a capital gains tax."
He said a tax on nitrogen losses would require the use of the Overseer modelling programme - but it had a 20 per cent margin of error.
"How many people would like IRD applying a 20 per cent margin of error to their taxes. Overseer is a fantastic tool for what it was developed for, but it wasn't designed as a tax calculator.
"Also, nitrogen is not the issue in every catchment in this country. Taxing every farmer and grower on nitrogen losses or a tax on nitrogen fertiliser, just takes money away from primary producers that they could spend on the mitigations that are needed for their specific catchment."
The Government will reveal which parts of the working group's plan it will adopt in April.
Finance Minister Grant Robertson said they needed to fix the gaps in the tax system, but were also working in an MMP environment.
That meant they needed to build consensus with the Greens and NZ First.