The Heard Island and McDonald Islands (HIMI) are Australian external territories, and they do not have a unique flag of their own. They use the Australian national flag, which has the following properties:

1. Colors:

   • Blue (#00008B)
   • Red (#FF0000)
   • White (#FFFFFF)

2. Aspect ratio: 1:2

3. Geometric properties and measurements:

   • The flag is divided into three parts: the Union Jack, the Commonwealth Star, and the Southern Cross.
   • The Union Jack occupies the upper hoist quarter and is 1/4 of the flag's width and 1/2 of its length.
   • The Commonwealth Star is located in the lower hoist quarter and has seven points, with each point representing a state or territory of Australia. The diameter of the star is 3/20 of the flag's width.
   • The Southern Cross consists of five white stars on the blue field: four large stars with seven points each and one smaller star with five points. The stars are arranged in a kite-like pattern, with the smallest star at the center.

As the HIMI does not have a specific flag, there are no additional details or symbolism to describe.