Forever in SA.
Cheers,
Peter

From : https://www.qld.gov.au/transport/registration/roadworthy

How long are safety certificates valid?

The time a safety certificate is valid depends on who is selling the vehicle:

licensed motor dealerscertificates are valid for 3 months or 1,000km (whichever comes first) from the date of issue

all other sellerscertificates are valid for 2 months or 2,000km (whichever comes first) from the date of issue.

You can use the same safety certificate to register an unregistered vehicle and then transfer the registration, as long as you do this within the limits set out previously.

You will need to get a new safety certificate every time you sell a vehicle, even if you sell it within the limits set out previously. A single safety certificate can be used for 1 transfer only.

To the best of my knowledge, in Western Australia, Gas Certificate is forever, we do not have a RW (Road Worthy?) certificate over here

In Qld a new gas certificate is reqd when transferring ownership.. In Queensland, a caravan between 750kg-3.5 tonnes (ATM) needs a safety/roadworthy certificate when transferring ownership.