About 150 protesters blocked all the main entrances to the SkyCity convention centre in central Auckland today, where a petroleum industry conference was being held.
Continuously chanting and waving placards, they sat on the footpath in front of the centre, stopping anyone walking in or out.
About 20 protesters have been trespassed from SkyCity, but police said no arrests have been made.
There was a visible police presence behind each group of protesters.
Some delegates, including Energy Minister Simon Bridges, got into the conference centre earlier, but others have been unable to get in.
Protesters had also made it into the casino, and had attempted to get into the convention centre.
Greenpeace New Zealand invited the public to take part in "civil disobedience action en masse" against petroleum exploration, which they say damages the environment by accelerating climate change.
Executive director Russel Norman, who was part of the protest, said they were prepared to stay for a long time to get their message across.
"If you are part of the oil industry that's trying to destablise a stable climate - find more oil and burn it - then you are acting immorally and unethically, and we are going to sit here and make it difficult for you to go ahead with your conference."
Mr Bridges will today unveil the latest petroleum exploration programme, which he says is essential for the New Zealand economy.
Finding gas actually helped the environment because it displaced far dirtier coal, he said.