Mahe Drysdale admits there's a concern among some positives from his third placed finish in the single sculls at rowing's national championships on Lake Karapiro

The two-time Olympic champion has been well beaten by Robbie Manson, who held the single sculls berth for New Zealand at the last two world championships.

Manson has switched to the double this year, while Drysdale will take the single seat for two World Cups and likely the Olympics too.

Drysdale says he's still building his base for the year.