On Facebook, Matt Keith, 21, has posted the letter of apology he claims he was forced to write before the police let him and his girlfriend go.
Mr Keith posted: "Forced letter of apology before I could leave the police station...traumatising situation where it is apparently acceptable to be harassed, threatened and mobbed... I respect India and Hinduism completely. That's why I spent 35 hours getting a massive Ganesha put on my back and 4 hours getting the Goddess (Yellamma) on the only bit of space I had left on my body..my girlfriend ...does not deserve sexual abuse."
The young tourists were at a restaurant when a group came to their table and started harassing them about the tattoo on Mr Keith's leg and allegedly threatened to "skin him".
A policeman came to the spot but instead of helping the couple, allegedly told them off for the tattoo.
The tourists were then taken to the police station and allegedly detained for hours. Mr Keith alleges that for his freedom, he was forced to write an apology letter to his harassers.
A local politician denied the allegations and said that it was the tourists who had been rude. "We told him, please don't wear shorts, the tattoo can be seen. Navratri poojas are on and if he walked on the streets like this, he would have faced lots of difficulties. So we just asked him to wear pants and go around," said RV Ramesh Yadav.
A senior police officer, Sandeep Patil, said if it is found that the police forced the tourist to apologise, "the Bangalore city police will not tolerate such things and take very strict action."
The Australian High Commission, in a statement, expressed concern and said the consulate general is speaking to the authorities: "The Australian Government encourages Australians traveling abroad to research and respect local customs and laws," a spokesperson added.