Key said he had deliberately kept a low profile since handing over the reins to English.
"I wanted to make sure that Bill had some clear air. But I think all of us in this room would agree what an outstanding job he is doing as Prime Minister.
"I always had enormous faith when the transition took place that Bill would do a great job as leader of this country...it has been absolutely flawless and seamless."
The National-led Government on Thursday announced a target of raising the proportion of "swimmable" rivers in New Zealand to 90 per cent by 2040, at a cost of $2 billion to central and local government and farmers.
At present, 72 per cent of rivers, streams and lakes were considered suitable for swimming. Until now, the bottom line for water quality has been safe for wading and boating, not "full immersion" or swimming.
The policy was slammed by the Green Party, with its water spokeswoman Catherine Delahunty said a river with a grade of "excellent" could be "so polluted that 1 person in 20 gets sick from a waterborne disease".
Today, Key said the debate highlighted a difference between National and the Green Party and others.
"We are about practicality and not just ideology. The reality is you can't have every water way in New Zealand swimmable 100 per cent of the time, 24 hours a day, seven days a week.
"That's the difference between us - because we have been the long-term Government of New Zealand we've got a sense of what you can do, and the pace you can do it at. And the cost of that."
English said the "bit of a contest" over how to measure water quality data reflected the fact such work had not been done before.
"I know it is more challenging than duelling press releases, about who cares the most, but it is the way in which we will get progress in every waterway in New Zealand."
Meanwhile, the Government today released its conservation and environment priorities for the next 20 years, in a new "roadmap", produced by the Department of Conservation, Ministry for the Environment and Office of the Prime Minister's chief science advisor, Sir Peter Gluckman.
THE PRIME MINISTER'S ENVIRONMENTAL PHILOSOPHY
In his keynote address English spoke at length about his philosophy on the environment and how best to protect and improve it.
During his time as an MP representing communities in Southland, English said he saw a change in attitude from the Department of Conservation (DOC), "when they realised that they weren't the only ones who cared about the environment".
"Locals resented DoC's sense of ownership of anything that looked like it wasn't farming."
It was essential to engage communities through ambitious targets, such as those set as part of the Government's Pest Free policy, English said. Prices were another way to make progress, hence National's support for the Emissions Trading Scheme (ETS).
Environmental work was moving away from one of debate and conflict between environmentalists and Government and business, English said.
Many of those arguments had been won by environmental groups, the Prime Minister said. It was now better to have a broader approach that was "open-ended, community-driven, price-driven".
