Collecting multipolygons into a single multipolygon with ST_Collect() in PostGIS

How about using St_Union() instead?

CREATE TABLE new_table AS 
SELECT
  1::INT as id,
  ST_Union(geom)::geometry(MultiPolygonZ,2193)
FROM
  old_table

If this originates Topology problems (GEOSUnaryUnion: TopologyException) you can use ST_Collect() wrapped with a St_buffer() with 0 distance:

CREATE TABLE new_table AS 
SELECT
  1::INT as id,
  ST_Buffer(ST_Collect(geom),0)::geometry(MultiPolygonZ,2193)
FROM
  old_table