He was also the 2013 Australian 10km open water champion.
"The New Zealand series has always attracted me, but because I live on the West Coast of Australia it's been difficult to come, but this year I'm very excited to be racing," Mainstone said.
"Just don't label me the favourite as I have never raced in a wetsuit before and with tough competition from Kane and others, I think it could be anyone's," he said.
Mainstone is the latest high quality Australian to compete this summer.
He'll be hoping to join Jarrod Poort and George O'Brien, both not swimming this weekend, as winners this season.
"To be able to get someone who is top quality like Rhys definitely gives the series a massive boost." Radford said.
"The series has pulled in some really big names from Australia this summer and it's going to be interesting when all of us get in the water later on and put the heat on each other," he said.
Adding to the dimension of the clash between Radford and Mainstone this weekend is the fact they train together in Perth, are good mates and have had some royal duels over the years including last year's FINA World Championships in Barcelona.
Radford beat Mainstone at the world champs by just a tenth of a second in the 5km race.