Children will again be held in mainland detention centres, with authorities setting aside a compound at Darwin's Wickham Point centre for families.

It is a significant shift for Labor, which has maintained a long-time opposition to detaining children. Staff have been told that minors could be held within the centre by the end of the week.

A Department of Immigration spokeswoman said work had been done to make the compound suitable for families, who would be held separately to adult men.

Immigration Minister Brendan O'Connor has also confirmed he is considering sending children to a modified section of the Curtin detention centre in Western Australia.