Immigration Minister Peter Dutton has been hit with another class action lawsuit brought on behalf of asylum seekers detained under Labor and Coalition governments over the past six years.

Documents filed in the Federal Court allege a four-year-old boy, born in Darwin and detained on Christmas Island with his asylum seeker parents, was detained for no valid purpose and thus unlawfully.

Law firm Maurice Blackburn estimates "thousands" of asylum seekers are eligible to join the lawsuit, arguing their detention was unlawful because the government did not actively pursue their visa claims.

Mr Dutton would not comment on a matter before the courts, but has previously noted his department is the most litigated of all Commonwealth bureaucracies "due to Labor's failed border policies".