Police say protesters jumped a fence at the Waihopai spy base on Saturday afternoon to try to deliver a letter.
Members of the Anti-Bases Campaign believe the facility collects intelligence about New Zealanders, costs hundreds of millions of dollars to run, and needs to close.
The protesters met outside the base, near Blenheim, on Saturday morning and listened to speeches from four people.
A police spokesperson says officers told the protesters to leave or face arrest.
The spokesperson says the letter, which asked how many New Zealanders have been spied on illegally, was pushed into the face of a police officer, who then threw it to the ground.
No one was arrested, and the protesters left soon after.