The Takaka golfer, who opened with a superb seven under par 63, carded a five under par 65 in the benign afternoon conditions of the second round to post a 12 under par two round total of 128 to lead at the halfway stage.
He is four ahead of Taupo professional Troy Ropiha (67) with former European Tour professional Mark Brown (71) a further three shots back in a four-way tie for third place.
The Srixon Academy member was part of the New Zealand team earlier this year which won the Sloan Morpeth Trophy and claimed the leading individual at the SBS Invitational.
"It was always going to be tough to back up that round from yesterday," said the 22-year-old.
"I am stoked with the way I played today, particularly in the latter half of my round, to post five under today. It was awesome, a great feeling."
Riordan made birdies on holes five, six, seven, 10, 13 and 15.
If he won, he would become only the sixth amateur to win on the Charles Tour after Josh Geary, Troy Ropiha, Ben Campbell, Josh Munn and Fraser Wilkin.
Ropiha has also set his sights on joining an exclusive club in New Zealand golf.
The 26-year-old, who won the Taranaki Open as both an amateur and a professional, wants to become only the fourth golfer to win three titles on the Charles Tour.
He put himself in a good position today when he battled ferocious winds this morning to card a three under par 67 to back up his five under par start to post a 132 eight under par total.
Mark Purser, Michael Hendry and Doug Holloway are the only three-time winners on the Charles Tour since it began in 2004 and Ropiha wants to join them.
"I wouldn't mind jumping up there with those guys and win a third Charles Tour title that would mean a lot to me," Ropiha said.
Wellington professional Spearman-Burn carded a four under par 66 to climb 14 places on the leaderboard to a five under par total and within five shots of the lead
Brown was disappointed with his one over par 71 that saw him slip off the hot pace set by Riordan.
The men's cut was made at a five over par two round total of 145.
In the women's field, Bay of Plenty rep Grace Senior holds a one shot lead from her provincial team mate Heather Lavery after carding a three over par 75 in round two. Another Bay rep Susan Lines is a further shot back in third place.
The Tauranga golfer Senior will attract a good gallery over the weekend as she goes in pursuit of her first Charles Tour title.
