Natalie Lowrey arrived from Bali Sunday night to observe the court hearing scheduled for Tuesday of 15 Malaysians who were arrested along with her in June outside the plant of Australian miner Lynas Corp.
They have been charged with illegal assembly and rioting and if found guilty could be jailed for up to two years.
Australia-based Lowrey was refused entry to Malaysia on the grounds that she was blacklisted by police. She was detained at Kuala Lumpur International Airport for almost 15 hours before being put on a flight back to Bali early Monday, activists said.
Rights group Voice of the Malaysian People (Suaram) and anti-Lynas movement Himpunan Hijau in a joint statement condemned Lowrey's exclusion and criticised immigration authorities for allegedly denying the 40-year-old water for 13 hours.
Lowery was held by police for almost a week in June but escaped criminal charges.
Immigration authorities and police could not be reached for comment.
Activists say the rare earth plant in the eastern state of Pahang produces radioactive waste that threatens the environment and local people.
Environmentalist groups have staged a series of protests against the plant. Lynas insists it is safe, saying any radioactive waste would be low-level and safely disposed of.
Rare earths are vital for many industrial and hi-tech processes such as the production of smartphones, hybrid car batteries, wind turbines, steel and low-energy light bulbs.
The Australian miner hopes the plant can help break the Chinese stranglehold on the market for rare earths.
Lynas started processing rare earths in an industrial park outside the state capital, Kuantan, in late 2012 after a delay of more than a year because of strong local opposition.