Fitzgerald is in the process of applying to become a police recruit and said it is good to reach out to influencers – such as athletes – who could potentially also join the organisation.
Dagg, who has spoken of his interest in eventually pursuing a police career, normally posts behind-the-scenes rugby scenes and cute insights into his growing family.
He says he has no firm plans about what he will do on Monday.
"I'm just going to let it happen," he said.
"I'm really looking forward to it.
"People have their own perceptions of what policing is about but it's not often they get to see what really happens behind closed doors."
The Canterbury accounts have been teasing their mystery Instagrammer and on Friday social media regular Constable Ben Rutherford revealed Dagg's identity in a short video filmed at Rugby Park, Christchurch.
Instagram Takeover RevealINSTAGRAM TAKEOVER
Constable Ben let's us know who's taking over our Instagram account on Monday. Follow us on Instagram @canterburypolicenz and see what Izzy gets up to.
Hopefully we'll get the car keys back from Arlo by then.
#DaggSquad #IzzyTakesOver
Posted by Canterbury Police on Thursday, 7 June 2018
The initiative was aided by former Detective Superintendent Gini Le Bas, who left police to take up a role as personal development manager for the Crusaders franchise.
Inspector Hirone Waretini, who oversees the district's social media accounts, said the project was an example of high performance in action. "It's all about trusting our people," he said.
"For me, this is a great opportunity. We've got the Crusaders on our doorstep and that's a ready-made partnership – and we've got really good people who come up with really good ideas."