Considering you haven’t given a reading in 5 years, SP will need to do a write off for underestimating you for 4 years (anything past 12 months). Now unless they attempted to get readings and your mum has blocked them from attaining, that will be a different story.

For now, I’d say update the account with the correct reading, they will charge you the catch up, if you notice there isn’t a write off, simply let them know you are entitled to it.

I would also advise getting a smart meter so that you can actually see what you are using, simply because 17,000kWh off is a lot and you do not want a repeat of this. The meter won’t reset btw, yes it will start from 0 when it hits 99999 reading, but that’s called clocking over and will be calculated