Police have described the crash at the intersection of State Highway 1 and Tutukau Rd on Tuesday afternoon as "absolutely devastating".
Paul's daughter-in-law Tiffany, who did not want her last name published, said they had "been keeping really close together" since the tragedy.
"[We have been] sharing good memories, sharing funny memories and just making sure we are there for my partner [Paul's son Matiu] and his siblings and all our children who miss their nana, their cousin and their brother."
Tiffany said the boys' mum Susan was coping as well as she could "considering she lost her first born, her third born is in hospital and she just lost her mum".
Kaydentz was doing "extremely well" in Starship Hospital, she said.
Susan and her three sons were living with Paul in Kaikohe. Her middle son Nevada, 4, had not gone to Taupo with his nana and brothers.
The last time Tiffany saw her mother-in-law and nephews was on September 28, the day they started their journey to Taupo via Auckland for the celebration.
"Hone was extremely excited. He comes running up to me saying 'aunty, I'm going to Auckland and then I'm going to see my papa [Ahuriri]," Tiffany said.
Tiffany said everyone was wishing them a safe journey, while she made sure Hone had packed his PlayStation games.
"He loved his PlayStation that his papa Leighton bought him," she said.
Originally from Gisborne but now living in Australia, Ahuriri had flown to New Zealand last week for his birthday party on October 6. He then planned to holiday around the country and visit his children.
But four days after the party, when Ahuriri was travelling to Auckland with Paul and her grandsons - his plans came to a tragic end.
Paul had nine children, two of whom died at a young age, 16 grandchildren and an ever-growing whangai family.
"She was the cook, the baker, the one everyone rang when they were sick or when they lost someone," Tiffany said.
Hone was "extremely close" with his nana according to Tiffany.
"He called her mum," she said.
Paul and Hone have been taken to Mangamuka Marae in the Far North. Their funeral is set to be held on Sunday.
Ahuriri's daughter Kirsty Ahuriri, said the plan was to move her father to Gisborne today, where family would gather from around Australasia to lay him to rest.