They will send a letter and ask for your version of events. This could take a month (likely longer with the pandemic) to arrive.
If they intend to prosecute, they will list which offence you’re being charged under. On the surface it doesn’t seem like you have a defence to the relevant offences. It’s likely that a Regulation of Railway Act offence would be considered as what you did is worse than no holding a ticket at all and shows intent to avoid paying the correct fare.
My advice is to be highly apologetic in your response and ask if they would consider settling this out of court - be prepared to pay for the ticket alongside their admin costs.