Hey lubes,
Penrith can be pretty dangerous. As far as rapids go, it is probably one of the safest places (there are lifeguards standing at the ready, and no random debris floating in the water), but with hardly any experience, it is very difficult.
I think the bare minimum requirement should be that you are confident at rolling your boat, and quite strong with your paddling. I'll do my best to impart any extra knowledge I learnt from my one lesson I did years ago and improve on that basis

BTW, below is the trip I posted to the club:
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Ian and I will be heading out to Penrith on FRIDAY the 31st of October to do a few laps of the whitewater course.
If you want to join us, you need to be a strong swimming, able to eskimo roll, and/or be proficient on whitewater. Penrith is a Grade 3 rapid, and is very challenging.
We will be meeting at the Tram shed at about 10:30am, where we will select our boats and gear, and then drive out to penrith in convoy with anyone else who joins us. Entry to the course is $30, plus a $20 deposit for your number. I am not sure exactly how much club gear hire is for the kayaks+paddles+floatation+spraydeck+helmet + wetsuit(optional).
We will probably spend a bit of time practicing rolling on the calm water before anything else, but when it comes to 'practicing' on the rapids, it is all in or nothing - there aren't really any quiet bits to practice on...
It is feasible that people could come along and just spend the day playing on the flatwater section, practising rolling - but that will still cost you $30 entry to the park, plus club gear hire charges, just to play on flat water. I don't mind anyone coming along who wants to do that though.