Mills had posted a video of himself on Facebook riding inside a motorised submersible toy shark as it leaped and dived across Lake Wakatipu just 12 days before the attack. The video was for a "James Bond for a day" tourism promotion.
"We [did] some crazy jumps, it's a crazy feeling under the water," he says in the clip.
Catlins Surf School owner Nick Smart was surfing about 100m from Mills when the shark attacked. A holidaying off-duty doctor and nurse came to Mills' aid before emergency services arrived.
"The doctor told me [Mills said he] was sitting on the board and the shark bit his leg, so he punched it. He managed to stay on his board and catch a wave in ... you could see blood, and he was definitely in a bit of pain."
Mills was conscious as his partner sat with him telling him he would be all right, while several friends watched, Smart said.
"He was pale, but he had all his friends around."
Mills' surfboard had bite marks in it, which had been used to estimate the shark at 2m to 3m long.