megacity POM

You don't plan megacities to create. That's no different as the silliness the Pogerans had in thinking they could create a town.

Towns grow automatically and naturally in places that are strategic for moving wealth.

The largest cities are near the ocean because you can move wealth in infinite directions from an ocean port.

Port Moresby has never grown that large because the port isn't that good and POM certainly is not strategically located. Of course there is room for enhancing POM's attractiveness, but you can't make a megacity in a location that isn't very strategic. Like POM.