China have a hold on the last qualifying spot with a cumulative 78.02sec - 38.87 and 39.15 - while Jamaica will go to London as the No 1-ranked nation after a world record 37.04 at the world champs.
Millar has been having treatment on a dodgy hamstring, as has Harman, and said inching under 39 seconds would be a big ask.
"That's flying for us and we'll need perfect conditions and a perfect race with no mistakes to get close to it."
Millar can crack the 10sec barrier from a flying start but will need his teammates to get on board the express train as well.
Millar and Harman will be joined by Andy Kruy (Palmerston North), Tama Toki (Auckland), Isaac Tatoa (Auckland) and Carl Van der Speck (Auckland, now based in Sydney) in the relay set-up, with the four best on the day taking the start line.
New Zealand's last relay in Australia failed to crack 40sec and Harman admits 38.9sec will be a stretch. "We should smash 40sec because last time none of us had peaked but sub-39sec is pretty quick. If we all run PBs or quicker then we might get lucky but if not, it's going to be hard."