UNHCR's regional representative Thomas Albrecht said Australia should offer protection and respect to people arriving by boat.
He said the basic human right of every person to seek asylum from persecution is not diminished by their mode of arrival.
Mr Albrecht said those forced to flee persecution need and deserve conducive conditions of protection, and a sustainable long-term solution.
Meanwhile, the Australia-based Human Rights Law Centre says Canberra's proposed ban could affect 320 refugees already living there.
If enacted, the ban would be backdated to July 2013 targeting about 2,000 asylum seekers detained offshore by Australia on Nauru and Papua New Guinea's Manus Island.
The centre represents women and their families brought back to Australia from Nauru to give birth.
Its executive director, Hugh de Kretser, said 320 refugees now living in community detention, including 40 babies born in Australia, might never be allowed to settle.
"There's now 40 or so babies here and their families, in the Australian community taking their first steps in freedom," he explained. "But under Australian law they're at risk of imminent return (to Nauru).
"It's difficult to see how they can make that law even harsher, but it will certainly create further fear and uncertainty for families who have already endured so much."
Hugh de Kretser says the ban could also permanently seperate refugees in Australia from family members still on Nauru and Manus island.
Australia is trying to send the refugees to other countries but has rejected an offer from New Zealand.