A red flag's been raised over a lack of tug boats fit for work in Cook Strait.

Marlborough Mayor Nadine Taylor, and Greater Wellington Regional Chair Daran Ponter, have written to Transport Minister Michael Wood raising concerns.

In January, Interislander's Kaitaki drifted toward Wellington's South Coast after losing power with more than 800 people on board.

Ponter says it's likely the port's tugs couldn't have saved the ship if it hadn't restarted.

"They may have been able to hold it for a period, but ultimately had that continued, the ferry would have been on the rocks."

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