Not absolutely sure this is the right forum but here goes.
I was originally flying Singapore - Darwin Sunday 13th July at 10.25 pm, arriving 04.45 am. Yesterday Quantas sent me info that the flight was cancelled but I could rebook to Monday 14th July 7.20 pm, arriving Tuesday 15th July at 1.40 am.
The only problem is that I have booked a 3 day Kakadu trip leaving at 7 am Tuesday 15th July from a downtown hotel which cannot be changed. So I was a bit nervous about arriving so close to the Kakadu trip (what happens if there are any delays) as well as I was going to be pretty tired all of Tuesday. So after a call Quantas booked me on a fight to Darwin via Brisbane, leaving at Sunday 13th July at 9.05 pm, arriving at Brisbane Monday 14th July at 6.40 am, then leaving for Darwin at 8.20 am, arriving at 12.05 pm. All Quantas and plenty of time before the trip (19 hrs).

Now, I am wondering if I have made a mistake. Is 100 mins enough in Brisbane to get from the international terminal to the domestic terminal after I am presuming passport and immigration and checking in my suitcase on the next flight? What responsibility does Quantas take to make sure I can connect with the next flight?

the terminals at Brisbane are 2 kms apart. If your trips are on one ticket it will be Qantas's problem to get you there. I dont know what conditions are like at Brisbane airport but you could call Qantas and ask them if they think 100 mins is enough time. PS no U in Qantas

Will you be travelling in other countries before Singapore and Australia? Will you be checking luggage? Upon arrival in Brisbane, you proceed through immigration, collect your luggage if you've checked it, go through customs, exit the international terminal, board a shuttle bus that runs every 10-15 minutes (I just transited Brisbane 2 weeks ago), and then go through security in the domestic terminal. Whether or not 100 minutes will be enough depends on 1) how long you will need to wait to retrieve luggage and 2) how much time you spend at customs. When coming from the U.S. or NZ, it's a breeze for me, but when coming from Indonesia one time, the agents wanted to go through our bags searching for banned items as well as inspect our shoes for soil and bacteria. This does not consider any flight delays. Could be enough but it may not, but as northern says, as long as you're travelling on a single ticket, Qantas will likely be responsible to get you to Darwin.

Flying from Sweden straight to Singapore, arriving 11th July and then on to Australia. 2 checked in suitcases.

Here is the customs form that you have to complete upon landing. If you are able to answer No to questions 6 through 11, you should be able to get through customs fairly quickly. Hopefully you are able to retrieve your bags without delay.

https://www.abf.gov.au/entering-leav...le-english.pdf

So, the answer will be NO to questions 2-11. I am diabetic and have tablets with me and hopefully an English written letter explaining the need for my tablets (no insulin), should I answer YES on question1?

Yes if you have medications, but from firsthand experience, it will not hold you up.

Your only nonstop backup flight is ~12 hours later, but you have connection options:
Brisbane to Darwin | Google Flights

Thanks everyone. Have not had a chance to contact Qantas yet but I just had an update saying that the flight will now arrive in Brisbane att 6 am and the flight to Darwin will still leave at 8.20 am which means we now have 2.20 hrs to catch the next flight instead of 1.40 hrs.