"Thank you for your patience and understanding while we ensured everyone on site was kept safe from a potentially dangerous situation," the post said.
Whangaparāoa College principal Steve McCracken said the incident involved a past student.
The school was placed into lockdown at 2.45pm and the lockdown was lifted at 3.35pm.
Everything was back to normal now and school would resume tomorrow morning, he said.
Police said they received a report that a non-specific threat involving a firearm had been sent to some students at Whangaparāoa College.
The school was placed in lockdown as a precaution, police explained.
Enquiries identified the person involved in making the threat and police are speaking to those involved.
The firearm reportedly involved has been confirmed as an imitation firearm and will be destroyed, police said.
The young person involved will be referred to youth aid.