Heard Island and McDonald Islands do not have an official flag. However, there is an unofficial flag that is commonly used to represent the territory. The flag consists of a dark blue background with a white map of the territory in the center. The map is surrounded by a thin white circle, and above it, there are five white stars arranged in the shape of the Southern Cross constellation.

There are no standardized measurements, aspect ratios, or geometric properties for the unofficial flag of Heard Island and McDonald Islands. The color codes for the flag are not officially defined either, but they are commonly represented as:

• Dark Blue (#000080)
• White (#FFFFFF)

It's worth noting that the use of this flag is not officially recognized by the Australian government, which administers the territory.