Hopper describes Millane as "one of the most genuine people I was lucky enough to know".
"Anyone who has met her knows how sweet and kind she was," she wrote.
I can’t really process what has happened right now and I can’t believe Grace is really gone. My heart breaks every time...
Posted by Maddie Hopper on Sunday, 9 December 2018
"Grace never wanted anyone around her to be sad and she'd do anything she could to help make you feel better even if it meant not being able to talk about her own feelings.
"I just want to give you a big cuddle and never let go but instead I'm just going to pray that you're somewhere lovely and surrounded by dogs."
The post is accompanied by 10 photos of the friends together, and has received a stream of comments from other friends and people sending their condolences.
"Representing NZ and everything we are. We are truly truly sorry for your loss. Our entire nation is heart broken for all of you," one person commented.
Another said: "All New Zealanders have felt the heartache that this has caused and can only imagine the sadness and pain this has caused her family & friends."
Fr Jeff is at STMHS-We are about to offer Mass at school for the eternal soul of our former 6th form student , Grace...
Posted by St. Peter's Roman Catholic Church, Eastwood on Monday, 10 December 2018
The school both Hopper and Millane attended for their sixth form year has also posted a tribute.
St Thomas More High School in Essex wrote, "We are about to offer Mass at school for the eternal soul of our former 6th form student Grace Millane.
"The school community is in mourning - please pray for the family, Dad, David, Gill, Grace's Mum and their sons at this time of deep and tragic sadness."
Yesterday, Millane's brother Declan also took to Instagram to post a heart-breaking tribute.
"You are my sunshine, my only sunshine," he wrote beside of photo of him and Grace.
"You make me happy when skies are gray. You'll never know dear, how much I love you. Please don't take my sunshine away," he said, quoting lyrics from Johnny Cash's song You Are My Sunshine.
Millane's death has dominated New Zealand and foreign media since a missing person's report was filed on December 5.
Four days later her body was discovered in the Waitakere Ranges.
Since then, the country, friends and family have struggled to come to grips with her loss.
Vigils for Millane have been planned in Auckland, Queenstown, Christchurch and Wellington, with the first to be held in front of Queenstown's Boatshed Cafe at 7pm tonight.
