What punishment will I get from CASA if I fly in a cloud while a student pilot?
I have had a fair bit of experience with CASA, including a close friend who worked for them for a number of years. They have a bad reputation these days but I'm not sure it's entirely deserved. I obviously cannot predict exactly what they would do but I hope to give you some advice.

First things first, I would believe your version of events much more than someone on the ground - slant visibility and all that.  

The regulations (CAR 172) effectively states that if you fly into cloud the penalty is 50 penalty points - I think each penalty point is currently 110 dollars so the answer to your question is you could be fined $5500. However, CASA uses discretion as to when to enforce this fine - and the Department of Public Prosecutions gets the final say. If it is not a good use of their time, or if it doesn't really serve the public interest, they won't bother with it. A student pilot who accidentally flew into cloud? I would be very very surprised if they followed it up - particularly because as a student you are under the care of an instructor, who shouldn't let you fly solo if there was cloud in the immediate vicinity.

I don't think this would apply to your situation, but a more common course of action for CASA is the "fit and proper person" catch-all. Basically it means CASA must be comfortable that you will be responsible with the powers your licence gives you. CASA uses this power to remove the licences of people who frankly, shouldn't have one. It tends to be reserved for people who break the law either intentionally or through gross negligence. Also for example if you are charged with a non aviation offence, like fraud, CASA might take your licence claiming you are not a fit and proper person to hold one. 

Other items in CASA's arsenal include temporary licence suspension, forced re-training or re-sitting of an exam or flight test. There is also a 'demerit points' system if you are a repeat offender. 

Some general advice now (remembering that I'm not a lawyer) - don't lie. According to my friend (who should know), CASA is more concerned about your attitude than anything. They want to see people taking responsibility for their mistakes and accepting the consequences. This certainly doesn't assure you immunity but you might find that this reduces a licence suspension from 24 months to 6 months. If you are particularly worried, remember that you don't need to tell CASA anything if you are the person under investigation - like in other legal matters, you have the right to remain silent. Before any action is taken against you you will receive a "show cause" notice, which tells you what they are planning to do and invites you to tell them why they should not do that. If you don't like what they propose I'd advise upon getting a lawyer to respond on your behalf. 

But all pilots make mistakes and only 'temporary pilots' pretend otherwise. If you really did fly into that cloud, thoroughly debrief with your instructor and turn it into a learning opportunity. If CASA does come knocking then you can talk about what you have learned and that you are a better pilot for it. 

But, in the circumstances you've described, especially as a student pilot, if CASA comes down on you I would be questioning where my taxes are going. What you are going through must be intimidating but don't be fearful - if your goal is to get into the airlines you will quite likely experience this sort of situation again. As long as your intent was never to break the rules or put anyone in danger, you honestly should be fine.