2015 Hyundai Tucson Highlander Rusted Inside Petrol Tank

My issue is that my daughter complained to Hyundai in June about her car
having the following problem:
When doing 80km plus when you put your foot down on the accelerator it
loses power for a short period then takes off. When she took this to Hyundai
they could not fault the car and said everything is alright. Still having the
same problem she took the car back again and they said they checked
everything and the car is fine. Third time she took the car back and she
asked if she could take one of the Hyundai technician for a drive and showed
them the problem. The problem happened and the technician was very
surprised

The technician took the car back to the workshop and found the fuel tank
and filter cover with sand like substance. Hyundai conclusion was that my
daughter filled the car with contaminated petrol. The next day I went to the
workshop and could not believe what I saw. It was not sand instead the tank
was full of rust and the sand substance is the rust breaking down. I also
scratched at the tank and it was deeply etched into the metal.

Hyundai then changed their story and said the petrol is contaminated with
water and it will not be covered with warranty. What I cannot understand if it
she filled the car with a bad dose of fuel would the tank rust this quickly and
why could they not fault the car until she pointed out the issue.
I have complained to Hyundai with no result and have now taken our
problem to Department of Fair trading in Australia and we have a hearing
this coming Tuesday  25th July. My  daughter is not feeling confident after
speaking to them today. They seem to be convince about the water
contamination but not seeing our side about why the fuel tank has rusted so
fast and why was the car running faultlessly in accordance to Hyundai?  I am
convinced that the problem is a production problem not contaminated fuel.

Are you aware of any issues in regards to this that would help support our
defence.