Once copyright has expired, it cannot be revived by subsequent publication or in any similar way. Copyright can only be renewed or extended if the parliament amends the Act (as it was obliged to do as part of its obligations under the AUSFTA).
A publisher who publishes an edition of a “public domain” work (such as the poems of Keats) may own copyright in the typographical arrangement of that particular edition (and thus be able to prevent another publisher making an exact copy of that edition). However, the copyright in the work itself (in this case, the poems) is not revived by republishing; anyone can still reproduce all or parts of it.