German-born Joseph Kraus Karl was arrested at his house in Chiang Mai province on Tuesday after police received a complaint from the children's father.

The Thai girls, aged seven, 11, 12 and 14, were allegedly abused repeatedly after being invited to visit Karl's property in 2008.

"Initially he denied all charges and will only testify in the court," said police Colonel Kritapol Yeesakorn.

Karl, who faces a maximum of 20 years in prison if convicted, was born in Berlin.

He later obtained Australian citizenship but police said he has lived in Thailand for "many years" on a retiree visa.