The group of 11 were among 400 people part of a flotilla detained by Israel in international waters west of Cyprus early on Tuesday.
Video live-streamed to the Global Sumud Festival's website showed black-clad soldiers boarding the vessels while angles later taken by other flotilla participants revealed abandoned vessels bobbing in the sea as Palestinian flags fly in the wind.
Neve O'Connor, Sam Woripa Watson, Anny Mokotow, Isla Lamont, Juliet Lamont, Surya McEwen, Zack Schofield, Bianca Webb-Pullman, Gemma O'Toole, Violet Coco and Helen O'Sullivan were all confirmed to have been taken.
Their families have had zero contact with the detained activists for over 12 hours and don't know where they are.
"As a mother, it's the not knowing what's happening that is really terrifying," Ms O'Toole's mother Susie told reporters.
"I've been following this issue for a long time, and I know full well what the Israeli Defence Force are capable of.
"You're left to wonder and speculate at how low they might go."
Ms Webb-Pullman's mother Julie said her daughter had set off because she decided she didn't want to live in a world where Israel was not held accountable for their actions.
"If she dies in that effort, then that is something she's prepared to accept, and it's something I also must accept," she said.
The Israeli Defence Force's bombardment and starvation of Gaza has killed more than 72,000 people and destroyed the territory's infrastructure, according to the UN World Food Program.
Israel's foreign ministry has labelled the flotilla a publicity stunt and a "provocation for the sake of provocation", calling on all participants to immediately turn back.
The families have demanded Prime Minister Anthony Albanese publicly call for the detainees' release, end the two-way arms trade with Israel, expel its ambassador, and commit to arresting Israeli Prime Minister Benjamin Netanyahu should he set foot on Australian soil.
Four of the detained Australians were previously intercepted by Israel two weeks ago while three others were imprisoned in October.
The Department of Foreign Affairs and Trade is seeking confirmation of the intercepted Australians' welfare and has urged others not to join the flotillas for their safety.
"We will continue to make clear our expectation that any detainees receive humane treatment in line with international norms," a spokesperson said in a statement.
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