In an Australian referendum, what does a ‘double majority’ refer to?

• A majority of voters in a majority of states and across the nation must vote for the change
  Explanation:

  "A 'double majority' in an Australian referendum means that the proposal must be approved by a majority of voters in a majority of states (at least four out of six states), as well as a majority of voters across the entire nation."
• A majority of voters from each political party and in each state
• A majority of voters in each state and a majority of federal politicians must agree
• A majority of young and old voters must agree on the change