After the incident the Department of Conservation took his board away for testing and estimated that it was a 3.5m to 3.6m great white that attacked him.
Mr Mills, 30, was laying on his surf board when the shark came up to its side and bit straight through both the board and his leg.
"I didn't see it until it was biting me and then I was like, punching it, it was a bit strange. It was just instinct I think, to try and get it off you sort of thing.
"I still remember holding its nose, I reckon about for about three to four seconds but it felt a lot longer than that. It bit my surfboard and my leg at the same time. The bottom jaw went into my surfboard and the top jaw into the back of my leg cos I was laying down."
The shark eventually let go once it realised it was getting a beating, he said. He managed to paddle to the shore to safety.
"It just let go, I think it just realised that I wasn't food and it probably wasn't normal for itself to be getting hit."
Sharks cruising NZ waters
Sharks often make an appearance in New Zealand waters during the summer.
This summer there were three great white shark sightings near Auckland - with one witness estimated the creature weighed up to 800kg.
Department of Conservation shark expert Clinton Duffy has said there are about 71 species of shark in New Zealand, ranging from the 25cm pygmy shark to the 18m whale shark.
Thirteen were endemic to New Zealand and more than 60 per cent were deep-water species.
"In the upper North Island the shark most commonly seen from the shore off popular swimming beaches is the bronze whaler," Mr Duffy said.
The sharks most often seen "cruising" at the surface by people in boats were blue sharks and mako and in northern waters, boaties were also likely to come across hammerheads.
"The size of all three species generally increases the further from shore you get," Mr Duffy told the Herald earlier this year.
"Around most of New Zealand the sharks most often caught by people fishing from boats are spiny dogfishes, carpet sharks and school sharks.
"If you catch a shark and want to release it safely, avoid taking it out of the water if possible and cut the trace off as close to the hook as possible.
"Large sharks can be kept quiet at the boat by motoring it forward so that water continuously flows over their gills," he said.
Where you are likely to see a shark
"Most sharks will begin to suffocate as soon as they stop moving and will struggle to free themselves if restrained beside a stationary boat."
Boaties and swimmers were unlikely to chance upon aggressive sharks, which tended to stay deeper and further from shore.
Spearfishers were "most likely to have aggressive encounters with sharks because sharks are attracted and stimulated to feed by bleeding and struggling fish".
Mr Duffy said spearfishers could avoid attracting sharks by removing catch from the water as soon as it has been shot, or keep it on a long float line.
"If a shark shows up and wants your fish, let it have them, get out of the water and move location."
