A 13-year-old boy saved his younger brother from being swept out to sea in a daring ocean rescue, police say.

Senior Sergeant Anthony Bond said an 11-year-old boy was swept off the beach by a rogue wave at Toko Mouth, Chrystalls Beach, about 2.35pm on Saturday.

His older brother swam out to help him, then held on to him and swam back to shore, Snr Sgt Bond said.

Members of the public also got in the water to help.

Emergency services were called and a helicopter took the boys to Dunedin Hospital as a precaution. They were very cold and hypothermic, Snr Sgt Bond said.