According to the stuff.co.nz website, Collins remains in jail as police investigate why he was carrying the 17cm knife in a department store in the city of Hamamatsu at the weekend.
Kirwan has a lot of experience in Japan after playing and coaching there, including four years as Japan's national coach before taking over at the Blues this year.
Kirwan was quizzed about the matter when he fronted a Blues media session today and warned other situations involving rugby players in Japan had involved similar detentions.
"The Japanese are very, very strict on those types of things. You don't get a warning - you get put in jail. There is no leniency," Kirwan said.
"There have been a couple of situations where players have had little misdemeanours - what we would call a misdemeanour - and ended up in jail for two weeks."
Like most people following the case, Kirwan found the circumstances "bizarre". The former All Blacks wing had found Japan a safe place.
"I was really surprised. I don't know what's going on - it's a bit bizarre really - but you just need to be patient and make sure you go through the process properly. You don't want to get offside over there.
"Obviously, we're a little bit shocked - there's something going on, isn't there?
"I don't think it's my place to speculate what's going on. I've only read what you guys have reported really. I don't know him very well."