https://bitre.gov.au/statistics/aviation/international.aspx
Total seats made available on international scheduled operations to/from Australia during September 2015 were 3.643 million – an increase of 1.1 per cent compared to September 2014. The overall seat utilisation percentage (including China Airlines, Emirates and Qantas passengers travelling through Australian ports) increased from 79.0 per cent in September 2014 to 82.6 per cent in September 2015.
Emirates and QANTAS international seat availability accounted for 14% EACH of the total in Australia through Brisbane specifically.
That is to say, each airline moved a shade under 1M international pax through Brisbane over the same FY 2014-2015, according to the BITRE.
The answer for QANTAS international (how many billions) is ~1M pax for Brisbane and about 1.6M for Melbourne and a whopping 4.34M pax. There are other airports like Perth and Cairns but these are the big ones.
Total for QANTAS international overall is therefore is 0.007 BILLION available seats within a small error. In the total of South East Asia, BITRE noted there were 8.638M seats available for the year and all airlines. SE. Asia is by far the most popular international trip for people's going to and From Australia.
So without need of a really accurate estimate, 0.001-ish billion people are carried internationally by QANTAS through Brisbane which is almost irrelevant in comparison to the traffic through Sydney.
The most important city pair for QANTAS is Auckland Sydney then Sydney Singapore closely followed by Singapore Melbourne, Singapore Perth then Auckland Melbourne. Brisbane Auckland is eight on the list.
Top city pair all airlines for freight is Singapore Melbourne with Brisbane 6th on the list. Total airfreight for Australia for one year is around 1M tonnes
Conclusions:
QANTAS use big planes in slot constrained airports to move millions of people and thousands of tonnes of cargo between population centres annually. This requires dedicated infastructure and long term contracts with a number partners (ground handling, fuel, road transport, hotels, training, maintenance finance etc), implying a large sunk cost and high capital demand (actual real multiples of billions). Due to the size of these operations QANTAS and other airlines using the same infrastructure are neither agile nor protected from large unexpected changes in demand (SARS, 9/11 style terror fears, trade embargoes etc). However, the trend for freight and passenger movements internationally is generally up and is expected to be until another bombing/epidemic/volcanic eruption.
Wellcamp does not operate in the same space as Brisbane and does not compete with slot constrained airports for custom, international or otherwise. Any organisation such as an airline that is itself agile and not in need of specialist infrastructure, for example their own check in desk or containerised freight handling would have already run their ruler over Wellcamp to determine the relative merits of similar facilities especially if their typical trip origin is west of the great dividing range.
Brisbane does not have capacity to absorb a new airline for freight or passengers. Neither does Sydney.
Freight and passenger numbers for all of Australia are huge.
Wellcamp compares to Brisbane in the range of 1:1000 for passengers so the effect on Brisbane of doubling the number of passengers or freight through Wellcamp almost isn't measurable.
Brisbane isn't Wellcamp.
Wellcamp is irrelevant to Brisbane.
Brisbane and Wellcamp are almost irrelevant to the total of international travel from Australia.
Why do I need to count in Billions?
Why fly to Wellcamp for a coffee when I can drive there?
How many extra nurses will the $15M tax offset Wagners isn't paying actually pay for?
Please provide references for each answer but do please answer, lest the audience be left to assume the poster is a fool by their stern silence by way of a reply.