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A powdered substance sent to TVNZ and media firm Greenstone Pictures was almost certainly a common garden fertiliser, police said tonight.
Both companies received envelopes postmarked at the same place on the same day and containing an unknown powder.
The substance was sent to the NZ Defence Forces' Northern Improvised Explosive Device team for testing.
The team concluded it was "definitely not dangerous" and most likely to be powdered fertiliser, police said.
All staff at the Dominion Road Greenstone Pictures were earlier evacuated and staff underwent contamination showers after receiving envelopes containing an unknown substance.
Police cordoned off Ballantyne Square and the fire department's hazardous materials unit was sent to the scene.
TVNZ also received two envelopes containing a white powdery substance, though spokesperson Megan Richards said the powder looked "more like washing powder".
Ms Richards said police were called in to investigate and confirmed the powder to be harmless.
But police advised media outlets to report any suspicious mail "where the sender is purporting that an envelope contains anthrax".
There have been anthrax scares in New Zealand before but perhaps the largest letter threats have been cyanide-based.
In 2003, a national security alert was sparked after the discovery of a letter containing cyanide, considered a terrorist threat to the America's Cup.
Letters were sent to the United States Embassy and the British and Australian High Commissions at the same time. Two contained a powdered substance that police have since found to be harmless.
But the third contained cyanide.
A copy of the letter, also containing powder which later turned out be harmless, was received by the New Zealand Herald.
Police then believed the threats could be linked to a similar letter containing cyanide received at the US Embassy in 2002, threatening an attack on the NZ Golf Open, which featured Tiger Woods.
Anthrax is a popular poison for would-be criminals because of its deadly nature.
Inhalational anthrax, the strain most likely to be used by terrorists, leads to death in about 90 per cent of cases. Symptoms of fever and fatigue usually appear within a week of exposure.
Anthrax cannot spread from one person to another. Humans can be infected by three strains - inhalational (by breathing it in), intestinal (by eating contaminated meat) and cutaneous (usually through a cut on the skin).
Anthrax comes from a bacterium the size of a speck of dust.
The last human outbreaks in New Zealand are thought to have occurred about 100 years ago. The last cattle outbreak was in Te Awamutu in 1954, when 23 animals died.
