Mr Key was speaking to an audience of about 500 business people in Brisbane yesterday and mentioned Castle-Hughes while discussing climate change.
He asked if the audience had heard of her, mentioning her role in Whale Rider and also as an ambassador for Greenpeace's Sign On campaign, which is seeking a 40% reduction in emissions by 2020.
"My advice to Keisha is this: Stick to acting."
Mr Key also gave a blunt assessment of his expectations for next week's talks in Copenhagen.
When asked if he had any optimism for those talks, he replied: "Not really."
New Zealand will release its revised target for emission reductions by 2020 at that meeting - a target Mr Key said would balance economic reality with environmental need.
"There will be an outcome - I'm just not sure it will be in 2009 at Copenhagen."
He expected it would spill over into 2010, saying a watered-down commitment from developed countries was likely.
Castle-Hughes recently went to the Cook Islands to see the impact of climate change and returned saying she wanted to meet Mr Key to discuss it.
