Muir was left alone up front in the women's event as she strode home in 22m 55s for the 6km.
Triathlete Karen Toulmin has joined the Napier HC ranks this season and looked very strong to be comfortably second in 24m 43s ahead of her clubmate, Karen Moore.
The combined U16/U14 race produced an excellent competition in a classy field. Mitchell Snell put the foot down with Ronan Lee and Ollie Marshall clinging on.
The Napier trio slowly stretched to spread the field far over the 4km distance. In to the second lap and Snell had opened up a gap on Marshall with the long stride on the frontrunner eventually proving too much for the chasers.
Snell stopped the clock at 14m 18s with Marshall 18s away in second place.
Lee kept up a solid run to be 10s further back with Mark Day and Connor Craig the next home.
The first of the U14 runners was Hugo Lynch from Hastings in 16:25. He had a comfortable buffer over Napier runners Ethan Green and Johnathan Moore.
Brianna Lee was equally dominant in the G16 event with a gritty 16:26 for the 4km to have her more than a minute ahead of her Napier clubmates, Alia Wentz and Nina Boesch.
The U14 grade saw another Napier trifecta with Aniela Apperley comfortably claiming the win in 18:16 from Sophia Moore and Stella Marshall.
On a day where there were more than 100 competitors, the U12 event produced one of the biggest fields.
Tyler Waite had enough in the tank to hold off Oliver Berry and Robin Moore to win the 2km event in 8:13.
In the girls' event, Sian Llewellyn had to work hard to win from Pyper Jennings and Carys Llewellyn.
In the open 4km event, Richard Potts and Claudia Layton were victors while in the cubs it was Brooke Speers, McKenzie Speers and Millie Waite on the podium in that order.