I have noticed there is a large amount of steam coming from my outdoor Stellar hot water system; is this normal?

Exact model number of system?

It's a Rheem Stellar Mains Pressure Water System 120805AD

I do not find that number listed.

It seems you are from down under and I am located in the USA which means I have little experience with the exact system but from what you say about the steam it seems like your safety valve has opened and is discharging, The cause of this should be found right away and I will go as far as to say the system should be shut down until the problem is located.

Gas Water Heaters - Rheem