Police are cracking down on boat thieves but have warned Auckland owners to make sure their boats, particularly jetskis, are safe and secure.

Police said jetskis parked in highly visible areas are particularly vulnerable and many are stolen to order.

They said some jetskis were worth $25,000 and power boats could be worth $100,000 or more.

They were appealing to thieves who could be in and out within five minutes, even where a trailer was fitted with wheel clamps, padlocks or a security system.

The head of the Auckland police maritime unit, Senior Sergeant Martin Paget, said one owner recently disturbed thieves trying to steal his jetski.

The attempt failed but they returned the next night to try again but failed.

Mr Paget said that last year 110 boats were stolen in Greater Auckland, compared with the record of 350 four years ago.

He said a handful of people were responsible for a lot of the thefts and once a few had been charged the thefts tended to drop dramatically.

He said security systems were recommended but often only delayed a thief.

Thieves often cut off towballs fitted with security systems and fitted a new towball, or they would carry a wheel if a trailer had a wheel clamp.

He said a small jetski trailer could easily and quickly be loaded onto the back of a truck or ute.

He said boats and jetskis should be secured and parked in areas difficult for a thief to see or to remove a trailer from. Security lights or alarm systems were also recommended.