Since the attack on two mosques in Christchurch on March 15, tens of thousands of Kiwis have participated in vigils across the country, and overseas.
The Al Noor mosque, where 42 people were killed by the gunman, is opposite Hagley Park.
It was reopened yesterday.
"In the week since the unprecedented terror attack there has been an outpouring of grief and love in our country," Ardern said, this afternoon.
"The service will be a chance to once again show that New Zealanders are compassionate, inclusive and diverse and that we will protect those values."
The service will be a huge security and logistical operation, given the likely huge scale of the event.
Foreign leaders and dignitaries would likely be in attendance, as well as global leaders of the Muslim Community.
The Herald on Sunday has confirmed at least one member of the royal family would attend.
With Meghan, Duchess of Sussex, now heavily pregnant, it was likely Prince William would represent the family.
A spokesman for the Prime Minister said foreign dignitaries will indicate if they wish to attend.
Ardern said the Government was anticipating international guests and visitors will want to attend and to "join us in our sorrow and support for the victims and their families".
New Zealand's terror alert is still a "very high" – a level it has at been since the attack.
Christchurch Mayor Lianne Dalziel said the service will be a time for everyone, locally, nationally and internationally, to come together in unity.
"I am very proud of the way our city and nation have responded to this terrorist attack.
"What happened on Friday, March 15, 2019 will never define us. It is what has happened since, the love and compassion, that's what defines us.
She said that coming together on Friday will demonstrate support for all New Zealand's communities – "no matter where we were born, no matter where and how we choose to express our faith. We do not stand alone, we stand together".
