Calvert said although no individual player had been truly poor in the first half, collectively Whanganui had just struggled to get into the game, which can happen on the road.
Playing straight and then moving the ball into compact areas, they were too repetitive in their attack and easy to pick off.
Marist scored from a corner kick and then followed up swiftly to get their second goal five minutes later.
Having already made a tactical change with Saskia Knight coming off the bench, Calvert put fellow reserves Alice Hetherington and Emma Wood on at halftime, and from then on Whanganui were a different team.
"We were on the front foot, sharpened up our passing."
While Marist did get a couple of breakaway opportunities, Calvert said they weren't enough to be threatening, although he thought the game would now resemble last weekend's 3-1 loss to Hokowhitu FC, where Whanganui finished strongly but couldn't break through.
Like Hokowhitu, the opposition goalie received her team's player of the day award after multiple saves.
However, Whanganui finally closed the gap after an 80th minute goal by Hetherington, coming in on the angle.
Then in the final minute, captain and top scorer Vanya Apiata Hodgkinson got the equaliser for her fifth goal of the season.
"A complete carbon of the goal she got in the first game against them," said Calvert.
"[It was] their intensity in the second half to get on the ball and play – the bravery they showed."
While in April the coach had thought Whanganui deserved a win, with Marist just hanging on at the finish, it was probably fair for the teams to share a point each in the return game, given how the momentum swung between both halves.
"Quite a highlight of how even the League really is this year.
"We can't start games like we've been starting away from home."
Having kicked off the second round of League games, the draw moved Whanganui to a 1-4-1 record, sharing third on the table with Marist due to a matching points differential.
Second place is Taradale AFC, who had a 1-1 draw with Massey University FC on Sunday, while undefeated Hokowhitu FC continue to check out well ahead of the pack, after they hammered wooden-spooners Feilding United 7-0.
League play now goes on hiatus for the weekend as Whanganui enters the GR Engineering Federation Cup, hosting Massey at Wembley Park on Sunday at 1pm.
"We're excited for Fed Cup, first time the team's been in it," said Calvert.