The federal government insists Christmas Island will provide adequate health facilities after Labor asked for an explanation on how it would treat asylum seekers.
After the parliament passed laws making it easier for medical transfers from Manus Island and Nauru, the government announced it would reopen the Christmas Island detention centre which was mothballed last year.
Prime Minister Scott Morrison argues the reopening - at a cost of more than $1.4 billion - is a necessary step to deal with the expected influx of asylum seekers requesting medical transfers.
However, authorities on Christmas Island say there are barely enough health facilities on the island to cater for locals, let alone hundreds of asylum seekers with a wide variety of medical conditions.
A day after Labor leader Bill Shorten said reopening the detention centre was "fine" if medical treatment "makes people well", his deputy Tanya Plibersek said the government still has some explaining to do.
"It is up to the government to explain how people who cannot be adequately treated on Manus Island and Nauru can have adequate medical treatment on Christmas Island," Ms Plibersek told the National Press Club.
"It is up to them to explain how they justify the extra expense of doing it this way given that this government has already brought hundreds of people from Manus Island and Nauru to Australia to the mainland for medical treatment."
Immigration Minister David Coleman said on Wednesday there would be adequate facilities, but declined to comment on what was now available on the island.
"If a person needs to be treated for a particular matter, adequate facilities will be provided at Christmas Island," he told Sky News.
Ms Plibersek said she understood that in recent times about 400 people had been brought to the mainland for treatment, accompanied by about 500 family members.
"How can it be safe for those people to be brought here by Peter Dutton but we need to open Christmas Island for anybody else? I'd like the government to explain that."
Attorney-General Christian Porter told parliament Christmas Island was the "appropriate response" because there was limited capacity in onshore detention facilities to handle an influx.
As well there were "very real implications for our onshore justice system of transferring persons on to the mainland where we are aware that the person is charged or accused of a serious offence or we have reason for grounds to believe that they are of bad character", Mr Porter said.
