Riordan, who last week won the North Island Strokeplay Championship when that event was incorporated into the Charles Tour's Akarana Open, opened with a round of 64 to lead by one over Waikato's Compton Pikari.
Riordan, a quarter semi finalist at the 2013 Australian Amateur Championship, is from Takaka about an hour west of Nelson but spends a lot of time in Nelson as his coach Glyn Delaney is the professional there.
Unlike the case in previous years, the New Zealand Amateur Championship no longer attracts the leading Australian Amateurs, a disappointment perhaps given the number of Australians to have won the event over the years.
Matt Jager was the most recent Australian to win the event, the West Australian winning twice in 2009 and 2010.
In the New Zealand Women's Amateur Championship, also at the Nelson Golf Club, 17 year old Auckland player, Wenyung Keh, recorded an opening round of 69 to lead by two over her sister Munchin Keh, the 2012 New Zealand Amateur Champion.