"They (Napier City Rovers) put out a very strong team and we knew very quickly we were in for a game.
"Our performance was effective enough. We defended when we needed to and managed to get ourselves up the pitch."
Rovers scored late from a penalty to bring the deficit back to one, but Athletic held out for the win.
"It wasn't going to be the greatest-ever performance but I certainly felt from the body language of the players that it was a game we were never going to lose," Calvert said.
"Some of us have worked very hard for a number of years to get to that moment.
"We weren't going to let anything deny us of it on the weekend."
A win against Massey University this weekend would mean an unbeaten season, with a record of 12 wins and two draws.
Promotion to the New Zealand Football Central League beckons, but Athletic will have to get through a two-way playoff series against the winner of the Capital Premier League first.
The Capital Premier League continues for another month, with the playoff set for September.
The team was holding out for the trophy presentation until this weekend's home game, Calvert said.
"We were given the option of being given it up at Napier at the club rooms afterwards but I felt as though we should get it presented in Whanganui, in front of everyone who has supported us.
"It's something we should all celebrate together, as a club, a town and a region."
Calvert said the team would be back at training on Tuesday, and the beauty of the league success meant the season continued.
"We've had some fantastic moments so far. No one is going to be able to take them away from us or downplay what we've achieved.
"Our actions have backed up our words. I'm really proud and happy for the club."
The curtain-raiser for this weekend's game against Massey University will feature a local derby between Whanganui City and Whanganui Athletic Reserves.
Athletic Reserves are currently at the bottom of the Horizons Premiership table, with City two spots above.
"The lads are already very motivated for that, and there are always bragging rights if you can get one over on them," Calvert said.
"They (reserves) have had some ups and downs this season but they are putting in a lot of hard work.
"Hopefully that will come off in the next game."
Athletic plays Massey University at Wembley Park on Saturday.
Kickoff is 2.45pm.
