Police are reminding the public to prepare before going out on the water after a woman became stuck in a small inflatable boat in the middle of Lake Pukaki.

Police, Coastguard, an ambulance and a rescue helicopter were called to the alpine lake near Mt Cook about 7pm to rescue the woman, who had no means of getting to shore.

The woman and a friend were on the inflatable boat, without lifejackets, when it drifted into the middle of Lake Pukaki and then continued to drift.

The pair were only wearing swimwear and had no equipment to call for assistance or means of getting back.

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One of the people swam to shore to raise the alarm while the woman remained on the boat.

The woman on the boat was picked up by a Coastguard boat and taken to shore. She was then taken to hospital by rescue helicopter in a moderate condition.

Inspector Hirone Waretini said people need to make sure they have the right equipment before going out on the water.

"Especially flotation, warm clothing and some form of communication in case things go awry.

"This could have very easily not gone well for both women but thanks to the teamwork of our staff and colleagues in Coastguard and the other emergency services, everyone goes home safely.

"Things can change in a heartbeat so have fun and enjoy the water but be prepared, just in case."