Strong enough. The Dutch East Indies were right next door. Even though the Dutch did not have any settlements on New Guinea, there base of power is simply too close. So if someone else, like Leopold II decides he want all of New Guinea, the Dutch are in a position strong enough to quickly make a move to prevent anyone elses attempt to make a colony. Not unlike the way the British colonised Western Australia just before the French made their attempts (but easier since the Dutch were closer to New Guinea than the British were to Western Austrlia). That is of course besides the fact that generaly these claims were relatively well respected, especialy if it could be reasonably supported, like the Dutch and New Guinea.