Australian Simon Peter Carman "lived his normal life", going out, washing his clothes and talking to neighbours after allegedly murdering a teenage girl and stuffing her body inside a suitcase in Thailand, police say.

A man from Ballarat is in custody after the body of a teenage girl was found in a suitcase in Thailand.

Thai media is reporting that Simon Peter Carman, 46, was detained at Suvarnabhumi Airport in Bangkok while attempting to leave the country.