Being a "terrorist entity" means it is illegal for people to "participate in or support his activities".
Anyone who was "financially encouraging" the terrorist would be charged, Ardern said.
That means anyone who attempts to send him money will face the full force of the law.
"This designation ensures the offender cannot be involved in the financing of terrorism in the future."
The decision means Tarrant's assets – such as any bank accounts – have been frozen by officials.
While living in his native Australia Tarrant had told people he received a $500,000 inheritance when his father died and an investment in crypto-currency had allowed him to live without having to work.
He had also travelled extensively around Europe, visiting sites where historic battles had taken place.
Special security measures for locking up the mosque gunman will cost taxpayers $3.6 million over two years, a Cabinet paper released last month revealed.
The cost averages just under $4932 per day - compared to the average cost of $302 per day for incarcerating a standard prisoner.
Ardern said this afternoon: "We have an obligation to New Zealand and to the wider international community to prevent the financing of terrorist acts".
She added that the decision to put him on the list was made after his guilty sentencing last week.
But it was only announced this afternoon as Ardern wanted to make sure the victims of the attack had time to come to terms with the sentence.
Ardern didn't name Tarrant in her statement, and would only refer to him as "a terrorist entity".
Foreign Minister Winston Peters said Tarrant's crimes were some of the worst since World War II.
He said the new designation denies him, and his support base, any support.
"It's a matter of international common sense and co-operation."
Full list of all designated terrorist entities:
• Al-Aqsa Martyrs' Brigades (AAMB)
• Al-Shabaab
• Continuity Irish Republican Army (Cira)
• Ejercito de Liberacion Nacional (ELN)
• Euskadi Ta Askatasuna (Eta)
• Harakat-ul-Jihad-al-Islami, Bangladesh (Huji-B)
• Indian Mujahideen (IM)
• Islamic Resistance (IR)
• Islamic State of Iraq and the Levant - Sinai Province (Isil Sinai)
• Izz al-Din al-Qassam Brigades (IQB)
• Jaish Al-Adl
• Jamaah Anshrout Daulah
• Maute Group
• New People's Army/Communist Party of the Philippines (NPA/CPP)
• Palestinian Islamic Jihad (PIJ)
• Partiya Karkeren Kurdistan (PKK)
• Real Irish Republican Army (Rira)
• Revolutionary People's Liberation Party/Front (DHKP/C)
• Tarrant, Brenton Harrison
• The Shining Path