The Northland harbourmaster has ordered boats to keep 500 metres away from large ships in Whangarei harbour.
The special safety directive follows a near-miss between a kayak and an oil tanker approaching the Marsden Point oil refinery in May this year.
Jim Lyle said it's the latest in a series of dangerously close encounters on the harbour.
Captain Lyle says big tankers have a limited ability to manoevre. Small vessels risk being hit, or forcing the tankers to leave the marked channel, with dire results.
The 500-metre rule applies from Friday to all boats under 24 metres.