For why the miles were used its actually very simple.

The vast majority of development in the area occurred as a result of its usage in WW2 and of course as the govt center. And much on the infrastructure was built at the direction of the Australian military.

Thus planning, common names for hills for gun batteries, and such would all get names from them, at least they would be the ones recorded. And Australia had not adopted the metric system yet in the 1940's. Formal metrication would not begin until the mid 60's there. While we should also note it may not be statuary miles that the names originally were measured in, as a port, and airbase nautical miles are the common measurement for both and their own unique length separate from a 'normal' mile.

For more on the nature and extent of the construction, and especially the harbor defenses of Port Moresby you may want to check out this link: http://indicatorloops.com/portmoresby.htm