Is there something n the water Down Under? No sooner does New Zealand take a long step toward opting out of its ANZUS alliance with the United States and Australia than the Australian prime minister says he's awfully sorry but he can't make good on his commitment to let American aircraft use local bases while they're monitoring nearby MX missile tests. Robert Hawke's awkward little announcement, which events at home made him deliver while he was visiting in Washington, is far less serious than what the New Zealanders have done. But it reflects a similar nuclear allergy, and it is disturbing to see it on the loose.

In New Zealand, Prime Minister David Lange wants to have it both ways: to deny access to U.S. Navy ships because they might be nuclear-powered or nuclear-armed, and to remain a member in good standing of the basic security treaty. He and his supporters have come to see the American nuclear umbrella as a lightning rod likely to draw an attack on them in a crisis. They treat requests to participate routinely in the common defense as provocative forms of interference in their sovereign affairs.

Most Americans, we suspect, will wonder what happened to New Zealand's traditional sturdy commitment to the principle and practice of collective security. But it is not for Americans to force upon any ally an unwelcome measure or form of protection. There is no call for retaliation of any kind -- economic sanctions have, foolishly, been mentioned. But the United States cannot join Mr. Lange's little charade and pretend nothing has changed. What he proposes is freeloading. It flouts the political and moral requirements of alliance.

As a Pacific naval power, moreover, the United States cannot watch with indifference the spreading of different kinds of anti-nuclear entiment through a vast realm of the South Pacific -- no matter how tranquil and remote that region may now seem. As a world power, it must consider what conclusions may be drawn by allies elsewhere and by other groups waving the anti-nuclear banner as they observe the American embarrassment in New Zealand.

The immediate focus of concern is Australia. Prime Minister Hawke leads a Labor party with a left wing reinforced by a party called the Nuclear Disarmament Party. He has some troubles and, it is stated, he hopes to ease them by a propitious concession on the high-profile matter of supporting the MX tests. Washington is going along. Certainly it is far better that the matter of Australia's alliance participation be handled within its own political arena than to have it become a matter of diplomatic contention. We hope Mr. Hawke knows what he is doing.