Simplify a shapefile/polygons to reduce file size

Take the map shape file here

https://datapacks.censusdata.abs.gov.au/datapacks/
It could be Statistical Area 2 or 3. (Geography boundary: ESRI Shapefile)

and reduce the shape file size to under 50MB

Use a tool like QGIS or anyother recommended simplify geometry

Note, the reduced file must not loose any large country or state.
Also, Geojson format required.

I need to instructions, so I can repeat the steps

GIS experts preferred