Extract from Energy Safe Victoria
No, these handheld plug-in polarity testers don’t test for earth continuity or loop impedance. Only a trailing lead, run to each power point or electrical approved and calibrated testers should be used for testing.
You should check the power point for mechanical compliance, then test the polarity, the switch, and the earth continuity.
All electrical fittings and fixtures need to be checked, to the extent practical.
Generally, there should be very few scenarios where it is not practical to test power points or appliances. Examples of situations where it would not be practical could include where a power point was inaccessible due to being behind a heavy object that can’t be moved by one person, or where there was a light fitting that required a very high ladder to access e.g. >2.4m.