About 35 protesters gathered in front of the Millennium Hotel, carrying signs and flags, to take a stand against the oil and gas industry, and climate change inaction.
Protesters also unfurled a large banner from the Kawarau Bridge.
While the protest was mainly peaceful, police were talking to protesters after a window at the hotel was broken.
In a pointed display of their sentiment, protesters wrapped police tape bearing the words "climate crime scene'' around the front of the hotel.
Delegates were quickly ushered inside the building, as protesters shouted "shame'' at them.
About 230 delegates were expected to attend the conference yesterday and today.
Oil Free Otago's Rosemary Penwarden said the conference was happening at a time when "millions of schoolchildren are pleading for a safe future''.
"Most ordinary New Zealand citizens want a safe future, they want a safe climate, and it is not OK to let these guys keep destroying the future for all of us.''
A couple of stony-faced security guards were stationed outside the hotel, but there was little police presence.
Otago-Lakes area commander Inspector Olaf Jensen said police were speaking to protesters about damage to a window of the hotel.
"The role of police is to ensure safety and uphold the law, while recognising the lawful right to protest,'' he said.
Millennium Hotel staff declined to comment.
Petroleum Exploration and Production Association of New Zealand chief executive Cameron Madgwick said protesters had been generally well-behaved.
"Their behaviour was good, they freely and generally peacefully expressed their views, and we completely support their right to do that.''
The protest had not caused any disruption to the event, he said.
At the conference itself, a policy plan was published on how the sector could deliver economic, environmental and social benefits for New Zealanders.
It included a proposal for a dedicated fund to invest the public proceeds from new oil and gas developments.
