An Aboriginal elder who has chained himself to a government building in Perth over his terminally-ill brother's imprisonment is entitled to a peaceful protest, police say.

Herbert Bropho - a key figure in last year's Heirisson Island "tent embassy" protests against a native title deal for the South West - has chained himself to Dumas House in West Perth, where the state government's main ministerial offices are situated.

He's seeking the release of his brother Richard from the maximum-security Casuarina Prison on compassionate grounds.

Richard Bropho, who was detained for failing to abide move-on notices, was last week diagnosed with liver and kidney failure.

A police spokeswoman said Herbert Bropho was not creating a disturbance and security staff at Dumas House were happy to have him there.

He was entitled to a peaceful protest, she said.