I will be flying from Darwin to Sydney, arriving 7:40pm. Is there enough time to collect bags, go to international airport, check in and board a flight to Hawaii at 9:40pm (2 hours later)?
If these are two different tickets, I would recommend against it.
While it's not impossible, it's tight and you carry the full risk of missing the connection. If your flight from Darwin is delayed or if the bags are slow to come out, you will basically be a "no show" for your second flight and you may have to buy a whole new ticket to Hawaii.
You have to carefully check the check-in, bag drop, and be-at-the gate cutoff times of the airline for your departing flight. Bag drops typically close 45 minutes to one hour before departure, but every airline is different. Also make sure you do online checkin upfront, so you don't have to deal with that at the airport and already have a boarding pass.
There is some info on connecting in Sydney here: https://www.sydneyairport.com.au/info-sheet/get-to-your-next-flight but it mostly says "Talk to your airline", which is exactly what you should do.