Public opinion about Australia's offshore detention policy is shifting, advocates say, as thousands of people attend large rallies around the country in support of asylum seekers.

Daniel Webb, of the Human Rights Law Centre, says doctors refusing to discharge baby Asha from the Lady Cilento hospital in Brisbane is just one example of the changing attitude.

"We have seen over 100 churches around Australia open their doors and offer sanctuary to people at risk of deportation," he told a vocal crowd gathered in Melbourne's CBD on Sunday.

"We've seen every single state premier, every single state premier, support calls for Malcolm Turnbull to show some compassion.

"And then there's you right now, around Australia, tens of thousands of people are standing together to demand better."

Organisers say more than 50,000 people across Australia turned out for the national Welcome Refugee rallies on Palm Sunday, calling for compassionate treatment of asylum seekers.

Protesters want 267 people brought from Manus Island or Nauru to Australia for medical treatment to be allowed to stay.

Eight-year-old Lela, in Brisbane, read out a letter she had written to former prime minister Tony Abbott.

"I think you should stop sending boats away from Australia and let them come, I feel sad about this," she said.

Protesters carrying "Free the refugees" placards and chanting slogans such as "Let them stay" also marched through Sydney's CBD.

Refugee Action Coalition spokesman Ian Rintoul said Australians have not forgotten any man, woman and child on Manus Island and Nauru.

"We will not rest until all of them are safe in Australia," he told the crowd to loud applause.

The rallies come as Parliamentary Budget Office calculations show closing down offshore centres and moving asylum seekers to the mainland for processing in the community would save the federal government $2.9 billion over four years.

However, changes to the arrival numbers of asylum seekers by boat would significantly alter the financial impact, the office says.

Greens senator Sarah Hanson-Young told the Melbourne rally that Australians were sick and tired of seeing children and families used as political footballs.