The data contradicts testimony from Department of Home Affairs officials that only a small number of people already in the country would be affected.

The proposed ban is targeted at the 3000 asylum seekers transferred to offshore processing centres since July 2013.

It would stop refugees resettled in third party countries like Canada or the US ever being able to get a visa to Australia, unless the minister granted them special dispensation.

Home Affairs officials have said the bill was required to stamp out a potential pathway into Australia, in light of the government's commitment that no one who came by boat would be permanently settled in Australia.

However, during a Senate inquiry into the bill, Greens senator Nick McKim pointed out not all people who came after that date were taken offshore.

Home Affairs regional processing and resettlement senior assistant secretary Alana Sullivan confirmed there would be a "small number" of people already in Australia affected by the bill.

"I wouldn't say 'a lot of people'," Ms Sullivan said.

But new figures reveal 2000 asylum seekers who came by boat were never transferred to offshore processing centres.

This is separate to the 1084 asylum seekers who have been brought to Australia for medical treatment from Manus and Nauru.

Senator McKim said the sheer number of people still in Australia undermined the government's argument for introducing the legislation.

"The government's continually arguing if they let any one of these people into Australia, it will destroy the integrity of our border protection system," he said.

"Now we know there's over 2000 people in this cohort who were never transferred offshore in the first place. It shows the arbitrary nature of the government's decision making and undermines the entire argument for their offshore detention regime."

When asked to explain the disparity, the department failed to respond.

However it told the committee in its submission the visa ban was not likely to lead to an increase in the number of people in immigration detention, as there was already a bar on that cohort applying for a visa.

The department also told the committee 14 families would be permanently separated under the proposed lifetime ban.

Their earlier evidence suggested between 10 and 20 families could be split as some members had arrived in Australia before the July 2013 cut-off.

However the number includes babies born to stateless parents in Australia who have attained Australian citizenship, and babies born to a permanent resident or citizen parent in Australia, the department confirmed.

Senator McKim said it was "horrific confirmation tearing families apart was official government policy".

The Senate committee is due to report by the end of next week on whether the bill should be passed.