A 12-year-old boy with health problems who went missing in Christchurch has been found safe and well.

Police earlier called for public help to find Alex Donaldson, who has the mental age of a 4-year-old, after he went missing from his parents' address in Broomfield.

They announced about 9.25pm that he had been found safe and well.

"We thank the public of Christchurch for their concern and for keeping an eye out for him," police said.