The opportunity to catch a Humpback Whale with it's mouth open in northern Australia is rare. It is only when the mouth is open that you can see the baleen of the whale that it uses to strain krill from the water.
In other parts of the world where Humpback Whales are actively feeding in groups the whales will rise to the surface from beneath with it's mouth wide open when it hits the surface, it then closes it's mouth and pushes the water through it's baleen which acts as a strainer separating the food from the water, the water passes through and the food stays behind and is consumed by the whale.
Sometimes a breaching or head lunging whale will drop it's bottom jaw before it hits the water but the only pictures I've managed to take of a humpback whale with it's mouth open is of baby humpbacks as they play alongside their mothers.
I have yet to see a feeding pod of mature Humpback Whales in Queensland waters.
As mentioned in the bubble screen section on this web site feeding Humpback Whales will co-operate when feeding and often use a bubble screen to surround small fish. They ball the fish tightly into an area surrounded by bubbles then rush from beneath with their massive mouths open engulfing the fish.