This is a possible site: SA Property and Planning Atlas. However (I now see) there is no copyright notice displayed - so best use the other explicitly CC licenced sites offered by the SA government.

I got tired of adding addresses to hundreds of houses, so reluctantly decided to research custom presets. What a pleasure it turned out to be - I recommend it. Here is all it took:

<?xml version="1.0" encoding="UTF-8"?>

<!--
* Put this file in ~/snap/josm/common/  [This is specific to Ubuntu Linux]
* Add to JOSM by selecting  Edit / Preferences / Tagging Presets
* and then press '+' on the top-right, then specify the path to this file.
-->

<presets 
  author="xrisnik" 
  version="1.0" 
  shortdescription="Presets File" 
  description="Setting residential building and address in JOSM">

  <item name="Residence + address" type="closedway">

    <label text="Edit"/>

    <space />

    <combo key="building" text="Residence type" values="detached,terrace,semidetached_house,apartments" display_values="Detached,Terrace,Semi,Apartments" default="detached"/>

    <text key="addr:housenumber" text="Number" default="" delete_if_empty="true"/>

    <text key="addr:street" text="Street" default="" delete_if_empty="true"/>

    <space />

  </item>

</presets>