A woman reported missing after she failed to return from a swim at a beach in the Firth of Thames last night has been found alive.

The 43-year-old woman was found "mildly hypothermic" about 11.45am near the Coromandel town of Te Puru, about 11km north of Thames on the Thames Coast Road, police spokesman Tony Sasso said.

Search and rescue teams failed to find the woman last night after she did not return from a swim at the Te Puru beach about 7.20pm.

About 20 to 30 personnel resumed the search at first light today.

Mr Sasso said the woman's disappearance may have been related to a medical issue.

Thames Coromandel sub area manager, Senior Sergeant Graham Shields, thanked the searchers and local community.

"This was a very positive result to find the missing woman alive. If it wasn't for the involvement of SAR volunteers and the public this could have had a very different result."