He said he was a fair way back in third place when he got off the bike to start the run but managed to chase down second place with about 3km to go.
"It's always nice to race in Rotorua. People were supporting me and a lot of people who knew me gave me time splits and encouraged me during the race."
Shaw finished the race in a time of 4h 20m, with Auckland's Josh Harrison winning in a time of 4h 19m 51s.
Race winner Harrison overtook third place-getter Matt Taylor during the last 20 to 30km on the bike, pulling away on the last lap of the run.
The women's race was not as tight, with Auckland's Anna Ross dominating the event in a time of 4h 39m 40s.
Completing her sixth Rotorua Half Ironman, Ross pulled away with a blistering bike leg and, once off the bike, her lead was never threatened despite a very fast run from Erin Furness, who came second.
This year saw the event combined with the Triathalon New Zealand Contact Tri Series, with 1000 competitors racing. About 400 competitors took part in the half ironman.
Race director Steve Nicholls was thrilled with the half ironman race which saw new bike and run legs added this year.
"This has to be one of the toughest but most beautiful half ironman courses in the country," he said.
"It was also great to see so many people at the venue, and by combining the Contact Tri Series with the Half Ironman we really showcased the sport to a wide audience."