Melbourne University student Sutha Thanabalasingam, 27, had been fasting for four days as part of a global protest aimed at brokering a ceasefire between the Sri Lankan army and the Tamil Tigers.

The protest began as a three-man hunger strike in Parramatta, in Sydney's west, on Saturday but moved to the prime minister's Canberra residence on Wednesday amid reports the Sri Lankan government had broken into the "no-fire zone" in the island nation.

A protest spokeswoman, Geetha Mano, said Mr Thanabalasingam, who has gone without food and water for about 125 hours, was taken to Canberra Hospital at 11pm (AEST) on Wednesday in a critical condition.

"He was determined to go on," Ms Mano said.

"It's come to this extent in terms of raising awareness of the situation in north-east Sri Lanka."

A volunteer doctor with the protesters, Sam Pari, said medical help was sought after blood tests outside the Lodge showed Mr Thanabalasingam's muscles were breaking down "quite rapidly".

Dr Pari said the hunger striker was initially reluctant to seek medical help but was convinced he needed to stay alive to tell the public about his family trapped in the conflict zone.

The Sri Lankan government says it is in the final stages of defeating the separatist Liberation Tigers of Tamil Eelam, who launched a campaign in 1972 to create a separate Tamil homeland on the island.

The remaining Tigers are trapped in the "no-fire" zone, in the island's northeast, along with thousands of civilians.

But Colombo is under pressure to agree to a ceasefire, after claims that 3,500 civilians have been killed in the first three months of 2009.

Five male hunger strikers - students aged in their twenties and thirties - are still camped outside the Lodge, alongside about 50 Tamil supporters.

They want Mr Rudd to apply diplomatic pressure to Sri Lanka to broker a ceasefire with the Tamil Tigers.