Ardern said a "significant delegation" from Australia would attend the service tomorrow, including Prime Minister Scott Morrison, Governor-General Sir Peter Cosgrove and Opposition leader Bill Shorten.
She said she has been in contact with Morrison frequently since the terrorist attack on March 15.
"New Zealand is incredibly grateful for the support of Australia, including the provision of 85 police staff and victim identification staff to help in what is our country's largest-ever police investigation."
Ardern and Morrison will be holding talks with both Morrison and Shorten after the service tomorrow, but she declined to say today what they might discuss.
Morrison has taken strong action in his own country following the attacks, calling for the Turkish Ambassador to Australia to explain President Recep Erdogan's Anzac references in the aftermath. He has also proposed measures to deal with social media companies that do not move fast enough against hate speech and extremism on their platforms.
Ardern said Pacific leaders would also be at tomorrow's service, including the President of Fiji Jioji Konrote, the Samoan Head of State Va'aletoa Sualauvi II, President of French Polynesia Édouard Fritch, Cook Islands Prime Minister Heny Puna and Ulu of Tokolau Afega Gaualofa.
She said 59 countries were sending diplomatic representatives to the service. The Organisation of Islamic Co-operation and the UNHCR were also sending representatives.
"We are very grateful for the international community's support and messages of solidarity which highlights the significance of this tragedy for so many people and countries ," Ardern said.
The Government announced this morning that the Duke of Cambridge, Prince William, would visit Christchurch in late April. It was not know whether he would bring his wife Catherine, the Duchess of Cambridge, or the couple's three children.
Ardern said the Prince had a strong connection to Christchurch, having visited after the Canterbury earthquakes, and he was keen to show his support for the city and the Muslim community.
Ardern also said the details of a Royal Commission of Inquiry into events leading up to the attack, announced earlier this week, were being finalised.
That included the terms of reference and who would lead it.
The inquiry will look at what could have or should have been done to prevent the attack.
It would look at the alleged gunman and his activities before the attack.
It will look at the SIS, GCSB, Police, Customs, Immigration and any other relevant agencies, as well as the role of social media
It will look at events leading up to the attack but not the immediate response. The work of first responders will be looked at separately.
The Government has also announced sweeping changes to gun control following the terror attack.
They include a ban on all military style semi-automatics (MSSA) and assault rifles, and related parts used to convert them into MSSAs.
A buyback scheme is also being developed.
Ardern said today the Government intended introducing the legislation in Parliament next week.
The Government will consider other measures in coming weeks.
Ardern said New Zealand was at the beginning of a new journey.
"We have never been free of racism. We have never been free of violent ideology. But our overriding values are ones of fairness, compassion and diversity.
"Government is not the only one to play a role in ensuring these values are upheld, but we do have a leadership role in this space.
"I don't have all the answers now but I am committed to finding them."
As well as Ardern, one of the survivors of the attacks will play a central role in the service.
Farid Ahmed, whose wife Husna Ahmed was killed in Al Noor Mosque, will speak at the Hagley Park service.
Husna had rescued children in the mosque from the gunman but was killed when she returned to find her husband, who was in a wheelchair.
Also scheduled to speak are Muslim leaders Shaggaf Khan and Mustafa Farouk, Governor-General Dame Patsy Reddy, and Christchurch Mayor Lianne Dalziel.
British singer Yusuf Islam, also known as Cat Stevens, will perform, as will New Zealand musicians Marlon Williams, Hollie Smith, Teeks and Maisey Rika.
The names of each of the 50 victims of the terrorist attack will be read by members of the Muslim community.
The service will be jointly led by the Government, the City of Christchurch, Ngāi Tahu and the Muslim community.
The live broadcast of the Christchurch remembrance service will start at 10am. It will screen live on nzherald.co.nz.
