As soon as may be after the publication of the notification under section 3, the competent authority shall cause to be served by registered post on the owner of the land and also on the occupier in cases where the owner is not in occupation of the land or, where the person to be served is not readily traceable or the ownership of the land is in dispute, shall publish in the Official Gazette, a notice stating the particulars specified in subsection (1) of section 3.