This could include increasing flows in small streams, the main rivers and Lake Whatuma, and opening and enlarging the river mouths.
"This is specifically focused on environmental work, it's a separate cost from what the council has already invested," Ms Lambert said in reference to the $80 million already committed.
If the council did not use all the water agreed to, once it had started paying for it, it could on-sell it, she added.
The alternative would be to go through this process every year when possible projects came up, in which case the council would need to buy the water on the spot market.
"This would be far less easy to plan for and there would be no guarantee that water was available."
One person asked why the council was not instead using surplus water in the early years of the scheme to evaluate what environmental gains could be made so they could make an informed decision on how much to purchase. "Instead we are being asked to blindly purchase four million cubic metres a year with no supporting data behind it. Paying for something you don't need or more than you need is not a saving."
Ms Lambert said the council's science programmes needed long-term certainty, and that it provided scope for more than one project to be considered.
"Otherwise there's the risk that a popular project could come up and we would just buy water for that but overlook others that are equally as valuable."
Modelling also showed that by year 10 the regional council would be getting dividends back from its investment, so the cost of $940,000 a year for the water would be more than covered by income from the dam.
Another public meeting is being held in Wairoa today, and the proposals are open to submissions until Friday, May 13.