Can I use an RSA certificate gained in NSW, in QLD and vice versa?

NO.  Each State has different requirements in relation to RSA.  NSW for example, do not recognise RSA certificates from any other state.  To work in the sale, supply & service of alcohol in NSW you must complete a face to face or online course provided by a training organisation approved to deliver RSA by the NSW Office of Liquor Gaming & Racing.  CHS Training is an OLGR approved provider of both forms of RSA training.