The government's financial and economic advisor, the Treasury, basically supports the principle of the government's water reforms, material acquired by RNZ has shown.
However, Treasury was uncertain about how well the plans would work and said there were problems with complex and even unavailable data.
The government proposed several measures in September to clean up New Zealand waterways.
The Action Plan for Healthy Waterways would impose an immediate halt on land intensification, install long term controls via local government from 2025 on, put tougher limits on the leaching of nitrates or phosphates into waterways, and other measures.
When releasing the proposals, the government said it wanted to improve water quality while not harming the economy.
Several farming bodies have suggested this was a forlorn hope, and could even cost the country $6 billion a year by 2050.
But information acquired via the Official Information Act showed Treasury endorsed the proposal.
"We support this paper," Treasury said in several briefings to Cabinet.
"There are a range of human activities in rural and urban environments, such as agricultural intensification and urban growth, that are polluting waterways.
"These activities are not internalising their environmental costs, and regional planning is neither sufficient nor fast enough to stop further degradation of fresh water and its eco-systems."
But the full extent of Treasury's support for the proposals was impossible to gauge, because of many pages of material redacted from the dossier.
Some early briefings gave a clue that Treasury was worried about the details even as it supported the principle of the government's action plan.
"There is uncertainty around the impacts of the proposals," Treasury wrote.
"There are a range of options being consulted on, some of which were developed quite late in the policy process."
It went on, adding there were "issues around data availability and complexity".
In one telling comment, Treasury suggested the government was rushing through its reforms too fast.
"The policy development process, and external timing drivers, meant that agencies have not had sufficient time to undertake analysis prior to the upcoming Cabinet decisions," Treasury wrote.
In another telling comment, Treasury repeated that the scheme would not cost the government any extra money, saying the $229.2 million Productive and Sustainable Land Use Package announced in the budget would pay for the cost of implementing these ideas.
This contrast with complaints from farmers that the scheme would cost them a lot.