The tidal streams in and around Cook Strait are unreliable and mariners are warned to exercise every precaution when navigating in the vicinity.

The streams often run in one direction for eight to 10 hours, but cases have been reported of them going for 18 hours or more. When the streams have been running in one direction for an extended period, it has been found that the opposite stream is much weaker or, on some occasions, hardly noticeable.

The maximum rates shown on the Cook Strait charts which are normally attained during spring tide conditions are also liable to be encountered at any other time. In the vicinity of Karori Rock and Cape Terawhiti rates of up to 7 knots are frequently experienced, but as a rule do not last for more than about an hour. Small vessels are warned to keep well clear of tide rips as they may lose steerage way and may, in extreme cases, capsize.