"It was really cool, a pretty breath-taking scene", Martell told the Herald.
"One minute you're eating your raspberries and then 'wow, that's a big fish'."
No one was in the water at the time, although a couple of other swimmers were at the other end of the beach, Martell said.
He is also a regular swimmer at the beach, and wasn't put off by today's sighting.
"I'll still swim there."
In the Papamoa attack, the man said the shark involved was a "baby" which "thankfully didn't latch on".
"By the time I came to the surface it was gone. I rubbed the water out of my eyes once I breached the surface and as I did I felt like a big rush of water, I assume from its tail, whip past me", he told Stuff.
In the DOC warning, people were told to avoid swimming in the main channels of Tauranga Harbour or fishing from kayaks and jet skis.
The public needed to remember they were sharing the coastal waters with a number of different shark species, DOC marine technical advisor Clinton Duffy said.
"There are always sharks around our coastline and at times they may come close to the shore."
There had been several confirmed and unconfirmed sightings of great white sharks in Tauranga Harbour recently.
"It is not unusual for them to be there, however when we're visiting the ocean, we need to be a little bit vigilant and aware of what's happening around us.
"Swim where there are surf lifesaving patrols, and don't swim or dive alone."
