It was "eminently sensible" to trade one piece of land for another if it led to a net conservation benefit, he said.
"The Supreme Court apparently, on the face of it, is telling us that that's not what the legislation lets you do."
DoC can already trade stewardship land - the least protected conservation land - for private land for many reasons, including mining, flooding and road-building.
But the court's ruling says DOC cannot always do the same on forest parks, or special protection area, which have greater protection under the law.
Forest & Bird chief executive Kevin Hague said the law changes proposed by English would be "a tax on the integrity of conservation legislation".
They would be opposed by the "entire environment sector" and would stoke a similar level of anger as the protests of mining on Schedule 4 land in 2010, he said.
Labour's Ikaroa Rawhiti MP Meka Whaitiri said her party was "absolutely" against any law change, saying it would be arrogant move and cause a huge public backlash.
Maori Party co-leader Marama Fox was more divided, saying her party could support the legislation at the first stage. She supported the decision to stop the dam from going ahead. But land swaps could also be beneficial to iwi, she said.
"Under Treaty claims, reserve land is quite often gifted back to Maori because that is the only land available. So it's not something we'd immediately vote against."
Conservation Minister Maggie Barry said any possible law changes would not be rushed. They would definitely not come before Parliament this term, she said.
Environmental policy lecturer Ann Brower, from Lincoln University, said the ruling had established that it was "not ok to 'un-conserve' land' with identified ecological values just with the stroke of a pen.
It would take a brave government to legislate over the top of the Supreme Court's decision, she said.