The attack took place at Bowentown Beach just after 5pm on Thursday and emergency services flooded the area soon afterwards.
The lifeguards who spotted Marlow were patrolling outside of the flags when they spotted her, Surf Lifesaving New Zealand's eastern region lifesaving manager Chaz Gibbons-Campbell says.
Matt Lawry says he looked-on as they worked to save Marlow and says a man was being comforted by an emergency services member nearby.
"Watching them try to save the young woman without success and seeing the man's white face was a very traumatic, harrowing experience," he said.
After CPR efforts stopped, a man walked about 50 to 70 metres into the sea and began splashing water over himself.
"It was harrowing and I can't stop thinking about the extreme grief I saw on his face as he left the beach," Lawry said.
Tadhg Stopford also saw the man walk into the ocean and says he waited in the deep for several minutes standing there.
"His entry into the sea was a challenge, I guess, to the shark who had stolen the life of his loved one … With my children around me, I felt his loss."
Earlier today, iwi from Otawhiwhi Marae installed a rāhui in the area which finishes on Friday next week, banning the collection of shellfish, all fishing, and swimming at the beach.
While the rāhui will remain in place, swimming will be permitted between the flags in certain areas of the beach from tomorrow.
The decision to open areas of the water was made by local kaumātua (elders), Waihi Beach Surf Club officials, Bay of Plenty Harbourmaster manager Jon Jon Peters, and Surf Lifesaving NZ and Western Bay of Plenty District Council representatives.
Signage will be erected to assist the public as to where they can swim.
Gibbons-Campbell said lifeguards from Mount Maunganui and Pukehina Beach would also be assisting to patrol along the beach over the weekend and into next week.