The mother of an Australian activist detained in Israel says her son is being abused, including having his head slammed into concrete.

Surya McEwen was one of half a dozen Australians arrested by Israel when a humanitarian aid flotilla bound for Gaza was intercepted.

Hundreds of people, including Swedish activist Greta Thunberg, tried to sail to Gaza to deliver aid to civilians facing starvation and famine as Israel throttled supplies and the humanitarian crisis is at catastrophic levels.

Jacinta McEwen said she was distressed at her son's treatment.

Ms McEwen said her son was on a hunger strike and the water he was forced to drink was "undrinkable".

"He's being maltreated. He's being intimidated. Threats have been made," she told AAP.

"He's had his arm dislocated and his head shoved

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