The Kumeu Militaria Show is an annual event which has been running for the past five years at the Kumeu Showgrounds and is known to sell a range of firearms.
Vendors of the show should contact event organisers for refunds, the post said.
Event organiser Danny Paton did not want to answer the Herald questions this morning.
In a written statement, Paton said cancelling the show was a mark of respect to those grieving.
"As the organisers of the Kumeu Militaria Show, we are appalled and horrified at the stupid, senseless Christchurch shootings," he said.
"The victims of this tragic event have our deepest condolences and our thoughts go out to all those who are suffering from it. As a mark of utmost respect to those now grieving, we have made the decision to cancel the show, which is recognised as perhaps the biggest in the country – even though thousands of people annually attend."
Paton acknowledged organisers of the event were "out of pocket" following the decision but would not share figures.
A spokeswoman from Kumeu Showgrounds said she agreed with the decision to cancel the show which was scheduled for this coming Saturday.
Reaction to the announcement has been mixed on Facebook.
One user said the decision was stupid, while another said: "Every time an event is cancelled after a terrorist event, they win."
Another person said the decision was a "bit of a pain" and that he was on the hunt for ammunition for a rifle.
"All the angry reacts and "stupid" comments are the reason why the general public don't give a toss about law-abiding firearms owners," another user wrote.
Most posts, however, seemed to be in support of cancelling the show.
"This shows great responsibility and genuine respect to the sad events that have happened. So unfortunate, but definitely the best thing to do given the circumstances."
According to Small Arms Survey, there are 1.2 million civilian-owned firearms registered in New Zealand - the equivalent of one gun for every four people.
There are also approximately 12,000 unregistered firearms in the country.