He told his friends as they tried to save him: "I love you guys, I love you," the Sydney Morning Herald reported.
Hundreds of surfers and bodyboarders gathered at Town Beach, in Port Macquarie, after the service. They joined in prayer before paddling into the ocean for a minute's silence, the paper said.
The deadly attack came a week after a surfer was killed by a shark off Gracetown in Western Australia.
Beaches from Sawtell, just south of Coffs Harbour, to Woolgoolga, further north, remained closed yesterday.
Police say the search for the shark resumed at first light.
Coffs Harbour police inspector Joanne Reid praised Zac's friends for their bravery.
"His mates struggled with the shark for a short time, then the shark let go and his friends courageously managed to get him to shore," she told reporters at the scene on Saturday.
But she told Sky News it took time and Zac lost a lot of blood on the way.
Surf Life Saving NSW manager Andy Kent said it was an "awful tragedy".
"The hearts of all surf lifesavers go out to the family of this young victim," he said.
Coffs Harbour's chief lifeguard Greg Hackfath said shark attacks were highly unusual in the area.
"We've had two recorded shark attacks ... in 150 years of settlement," he told News Ltd.
"We see sharks but the common saying is that you don't worry about them because they're well fed."
Friends of Zac - and strangers - took to Facebook to pay tribute to the teenager. A tribute page to Young, In Memory of ZAC, had attracted more than 18,000 likes
Through a series of comments and photographs, his former schoolmates and friends gave an insight into the teen's love of surfing and caring nature.
"Zac was always looking out for others. It didn't matter if he was busy, he always had time for those in need. It didn't matter who you were, Zac considered you a friend," Brandon Erga, a pastor at Port Macquarie's Grace Church, wrote.
"Your house was always open to anyone and everyone. I love you so much brother. I know I will see you again."
- AAP
