Police pushed through demonstrators and cut a chain on a door to get delegates into the New Zealand Minerals Forum in Dunedin.
A coalition of about 10 different organisations are holding the protest to disrupt the annual New Zealand Minerals Forum organised by mining lobbying group Straterra.
About 50 protesters chained a door and others sat down and linked arms to prevent many of the delegates entering the building.
Police pushed through the protesters to get delegates inside, but the barricade around the protesters was soon back in place.
Protesters began singing 'All Star' by Smashmouth, and one of the protesters superglued their hands onto doors into the Dunedin Town Hall to prevent delegates from entering.
RNZ reporter Tess Brunton said groups of Minerals Forum delegates formed outside the different entrances, some calling the protest a nuisance, while others said it would have no impact on their conference.
Police said three people had been arrested for breach of the peace but no one had been charged with anything.
The forum at the Dunedin Centre will hold discussions on a number of mining projects, including the next steps for the formerly state-owned Stockton Mine on the West Coast.
There will also be a session about the future of underground coal mining, with the agenda saying there is an opportunity for substantial undiscovered underground coal deposits.
Spokesperson for Stop the Minerals Forum Coalition Abby Spilg-Harris said earlier the protest was mainly against the forum's focus on coal, saying there was no place for the fossil fuel any more.
"We are against any further coal exploration and mining in New Zealand. After you've had the school strike for climate, you've had all of these amazing young people asking for the government to take action, and yet in 2019 we have all of these mineral and coal executives planning for more exploration."
Three ministers will be speaking at the forum over the two days. Energy and Resources Megan Minister Woods and Economic Development Minister David Parker will give an update on the government's approach to minerals and new technologies, while Regional Economic Development Minster Shane Jones will speak at a separate session about the future of mining.