An elderly Australian man who allegedly raped four young sisters in Thailand has died in jail in Chiang Mai.

Ninety-four-year-old Karl Joseph Kraus was arrested in 2010 in one of the worst cases of child sexual abuse ever uncovered in Thailand.

German born Kraus, a former railway worker from Sydney, was accused of abusing the four sisters for almost two years.

The youngest was aged five when the alleged abuse began.

Kraus pleaded not guilty to rape and sexual assault and was granted bail of $12,000.

He had attempted to have the charges against him dropped because of ill health.

His lawyer had argued that he suffers from depression and is also suffering from advanced prostate cancer.