Australian Capital Territory is about 1.5 times bigger than Oahu.

Oahu is approximately 1,545 km² (597 sq mi), while Australian Capital Territory is approximately 2,358 km² (910 sq mi), making Australian Capital Territory 53% larger than Oahu.
Tip: This to-scale comparison of Oahu vs. Australian Capital Territory uses the Mercator projection, which distorts the size of regions near the poles.
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