We did an overnight in Cairns with Celebrity and had a similar time frame as you. I'm not 100% sure of the name anymore, but I believe we did an off-ship reef excursion with a company named Reef Experience. It was a full day - they're all long trips! I can't tell you exactly what time we arrived at the cruise tender dock, but we had a 6pm sail away and made it back to the boat with time to spare. It's likely your ship will have shuttles waiting near the Cairns marina (an extremely short walk from where the reef tours dock) that you can easily hop on and get back to the boat with them. Celebrity did. Check with them on that! Hopping on the last cruise ship shuttle from town will alleviate any stress of getting stuck in traffic.
You'll want to find out the tender times from Norweigan as there is likely a set schedule and they probably won't make accommodations if you need to leave early in the morning.
We did a nighttime rainforest tour on day 1, so we wound up staying overnight at one of the hotels at the marina. Otherwise, I'm not sure we'd have made it to the reef tour in the morning! The ship docking times (and tender times) made it tough to do what we desired. However, if you're willing to swallow the bitter pill of paying for a hotel room, it will open more options for you. Cairns is freaking awesome! Have fun!