Air New Zealand started flying between Auckland and Chicago last November three times a week and the route has proved successful enough to step frequency up to five times a week at the end of this year.
The airline has said close to a third of its passengers flying across the Pacific to the US and Argentina come from Australia. Passengers use Auckland as a transit point rather than Sydney.
The first booking for Air NZ's Chicago service was from a Sydney passenger.
However, the Brisbane-Chicago service will keep more Australian passengers on Qantas, says House of Travel commercial director Brent Thomas.
Brisbane was a more convenient transit airport for Australians. Kiwis, especially those south of Auckland could also be tempted to dog-leg to Brisbane and on to Chicago - if the price was right, he said.
''The most important thing is scheduling and price.''
The Brisbane–Chicago service covers 14,326km flight will take approximately 16 hours 20 minutes - slightly more or less depending on winds. The Auckland-Chicago service is 13,183km.
When the Qantas service launches, it is expected to be the fourth-longest passenger flight in the world.
It will be the second longest flight on the Qantas network, closely behind Qantas' Perth-London service (14,499km), which the airline says has the highest customer satisfaction rating on its network.
Prices have not been set and seats for the new services are expected to go on sale within the next few weeks, pending final DOT approval.
Qantas and American Airlines say an expanded codeshare relationship and better schedules on transpacific services, opened up more connections to more destinations and reduced total travel time.
They say better access to seats on each carrier's network, leading to lower fares.
American Airlines also plans to open up more services across the Pacific.
The tie-up had been supported by the Australian Competition and Consumer Commission and the New Zealand Minister of Transport. Air New Zealand has a joint venture with United Airlines across the Pacific. The US carrier uses its planes to fly between Auckland and San Francisco.