I've just discovered that NIWA has been operating a new Mangere site since Sep 09 about 0.5 km SE of the previous one (officially opened on Oct 31). So it looks like the old site was retired on Dec 22.
NIWA's December 2018 National Climate Summary mentions that 224mm recorded at Mangere's new site (although I make it 225mm) was a Dec site record (as was Auckland Aero's 206mm).
Have also updated my Dec table to include the new Mangere site's figures.