Some additional background:

In the Australian parliament, the Senate is seldom controlled by the government and therefore the government must make deals with the opposition and minor parties to gain their support to pass laws. However, by convention, the opposition will pass supply bills related to the budget.

In this case, the opposition was refusing to pass supply bills. If those didn’t pass, the government would shut down, which was unacceptable.

The monarch’s representative, the governor General, has limited day-to-day powers. But they also have reserve powers that are usually theoretical.

In this case, the governor general decided to go rogue and rather than let the Prime Minister formulate a political solution to pass the bills, sacked the Prime Minister and put the opposition charge on the proviso that the supply bills would pass and an election would be immediately called.