McMillan says it's a reflection on the overall state of the sport.
"It's a chance to show how much depth we've got on in the country at the moment, the 4x200 is a really good indication. All the team's really young, I'm the veteran of the team real promising for the future and I'm excited for the Olympics."
Lauren Boyle's also qualified in the 200 freestyle, after making the 400 mark on day two, with Natalie Wiegersma adding the 200 individual medley to the 400 she achieved earlier this week.
Wiegersma's London hopes looked in doubt with a shoulder injury suffered last year, and says it's been an incredibly tough road to get to where she is.
"Very, very hard actually. I had a lot of obstacles to overcome. To be able to come here and qualify in both events, it's a big turnaround for me."
Having already qualified in the 400, Wiegersma's 2.13.17 in the 200 has seen her sneak inside the required mark.
She says knowing she's heading to London is a bit surreal.
"For the last two years it's been Olympics, Olympics, Olympics. Now that it's finally here it feels kind of strange. I don't think it's sunk in, maybe it will sink in later on."