A number of peace groups are protesting what they call a "weapons expo", saying there is "no room for the weapons industry" in a just and sustainable world.
But the New Zealand Defence Industry Association says it would be wrong to call the forum an arms or weapons conference.
"We would never hold a weapons expo on our shores. That is not what our forum is. You will find more ammunition and guns on display at the annual fieldays than what you will see at our event." association chairman Scott Arrell said.
More than 500 delegates from at least 150 organisations are expected at the Defence Industry and National Security Forum.
"People at our forum are maintenance business owners, ICT specialists, caterers and companies that provide footwear to our Defence Force and St John Ambulance staff."
Peace Action Wellington spokeswoman Jessie Dennis said the protesters were trying to block entrances to the building to halt "business as usual".
Representatives of some of the largest weapons manufacturers in the world would be attending today, she said.
"Profiting from violence is immoral. I think New Zealanders are really proud of the parts of our history where we've stood up for peace, so what everyone's doing here today is continuing that tradition."
Reverend Bruce Hamill was also protesting outside the conference.
"I'm here today to stop an arms conference, because basically these guys are trading in death and the weapons of killing," he said.
Hamill carried a sign that said "Vegas gunman didn't act alone". He said it referred to the "wider picture" of the people who supplied weapons and made them accessible to people such as the gunman, whose murderous rampage at a Las Vegas music festival last week claimed at least 58 lives.
Hamill said he also wanted to acknowledge the significance of Jesus "as a person who opposed violence".
"As a follower of Jesus, I wanted to join that cause."
Oil Free Wellington has slammed the expo, saying peace and climate issues are intertwined.
"You cannot have a just world where people make money out of the deaths of others," said Oil Free Wellington spokeswoman Frances Mountier.
"Just as climate change is being fuelled by oil and gas companies pursuing profit, war is fuelled by weapons manufacturers and industries seeking to make money. Climate change and war destroy homes, displace people and effect the poor and vulnerable.
"As climate change gets worse, so too will conflicts around resources, land, migration and displacement. Tomorrow we will be taking action against businesses which would seek to profit from such conflict and wars."
A group of protesters were arrested outside the 2015 expo, mainly on charges of trespassing. Those charges were dropped in court this year.
