Boyle started conservatively, finally hitting the lead at the 300m mark, before whizzing home in a slick 62.35sec to win in 4min 07.78sec. This was a touch outside the national record she set at the trials in April which she broke at Shanghai.
Kean, 19, from the Capital club in Wellington, showed his class to win the 100m backstroke, holding off a fierce challenge from Spaniard Juan Rando Galvez.
A finalist in the event in Shanghai, Kean led at the turn from compatriot Bassett but had to withstand a storming finish from Galvez to win in 54.71sec.
Bassett timed his finishing lunge to grab a dead-heat for third with Italy's Sebastiano Ranfagni.
Earlier Auckland's Neilsen Varoy (Roskill Magic) took seventh in the final of the 50m butterfly in 24.37sec, a touch outside his personal best in the heat.
Tash Hind finished strongly for sixth in the 100m freestyle. Hind (Capital) was seventh at the 50m mark but was strong coming home to clock 55.77sec for sixth in virtually the same time as in Shanghai, and just .39sec from a medal.
NZ results:
50m butterfly: Tim Phillips (USA) 23.51sec 1, Paolo Facchinelli (Italy) 23.85 2, Masayuki Kishida (Japan) 23.95 3. Also: Neilsen Varoy (NZ) 24.37 7.
400m freestyle: Lauren Boyle (NZ) 4min 07.78sec 1, Melania Costa Schmid (Spain) 4:07.97 2, Stephanie Peacock (USA) 4:10.25 3.
100m backstroke: Gareth Kean (NZ) 54.71sec 1, Juan Rando Galvez (Spain) 54.94 2, Kurt Bassett (NZ) and Sebastiano Ranfagni (Italy) 55.21 3 equal.
100m freestyle: Yi Tang (China) 54.24sec 1, Darya Stepanyuk (Ukraine) 55.32 2, Megan Romano (USA) 55.38 3. Also: Tash Hind (NZ) 55.77 6.
- NZPA