Somewhere in the 20-30kg range would be a safe bet - and most couriers won't check whether it's 25kg or 30kg
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I've posted a couple of 125 motors and they usually come in around 20kg inc packaging.
You can send a bigger motor through Auspost in two packages to get below the 23kg limit - bottom end in one, top end in the other. Don't know if this is still economically sensible since Auspost jacked their priced up.