Israeli naval officers are intercepting the Global Sumud Flotilla of more than 30 vessels attempting to reach the Palestinian territory.
Bianca Webb-Pullman was aboard Florida, one of the boats intercepted by Israeli forces about 2pm on Thursday (AEST).
Video from the boat shows naval officers boarding with weapons raised as life-jacket-clad passengers raise their hands.
Sydney man Abubakir Rafiq is believed to have been on another boat named Spectre intercepted earlier.
Juliet Lamont and Hamish Paterson were on the Wahoo, believed to still be sailing along with MiaMia, carrying Surya McEwen.
Dr Webb-Pullman sent a last message to her mother and threw her phone overboard before the Israeli interception.
"Even if I die, don't worry, it was still 100 per cent worth it," she wrote.
Her mother, Julie Webb-Pullman, has received updates from the flotilla's family liaison but said the Australian government's response had been "reprehensible".
"If she has to die because this government is not doing what it should be doing, then that's an indictment of the government and I will not forgive them," she told AAP.
Australians have been warned not to travel to Palestine since the October 7, 2023, attack by Hamas on Israel, which sparked the conflict engulfing Gaza.
The Department of Foreign Affairs and Trade has added specific advice on accessing the besieged strip by sea, advising of a naval blockade along the Gazan coast.
Israel has claimed the right to blockade Gaza for more than 15 years and its latest actions were not unexpected, international law of the sea expert Donald Rothwell told AAP.
But the size of the flotilla has presented logistical and operational challenges.
"There were so many vessels, obviously each one has to be individually boarded," the Australian National University professor said.
Another critical point was interceptions taking place in international waters, in violation of international law.
"Within those waters, the vessels in the flotilla were able to continue to exercise the freedom of navigation, irrespective of the status of the blockade,'' he said.
"The status of the blockade, legally, is questionable by many countries."
Any consequences would be dependent on action from the flag states of the intercepted vessels, including the UK, Italy, Turkey, Spain and Poland, Prof Rothwell said.
"One of those states might seek to legally challenge Israel's conduct."
That has not occurred with previous flotilla interceptions throughout the year.
"But they've all been single, one-off interdictions of vessels sailing on their own, albeit under this flotilla flag," Prof Rothwell said.
People detained would likely be taken to Israel and then deported.
"Israel doesn't really want to get caught up in the ongoing issues that would arise if they jail all of these people," he said.
To those in the flotilla, the desperate humanitarian landscape in Gaza makes the aid mission worthwhile.
Ms Lamont said about 30 Israeli vessels had surrounded the flotilla and were pounding boats with water cannons before attempting to board.
"It does seem at this stage they are unprepared to sink the vessels or to kill anyone," she told ABC Radio.
"We've just got to keep steadfast, keep dodging them, keep moving forward."
Israel's foreign ministry said several vessels had been "safely stopped" and passengers, including Swedish activist Greta Thunberg, had been transferred to an Israeli port.
"They are not interested in aid, but in provocation," the ministry said on social media.
