New Visa card didn't indicate any payment due but they still charged me interest

Absa issued me with a new Visa credit card a few weeks ago despite giving me no warning that this was happening. The first I heard of this was due to a courier phoning me to organise delivery of it and I, myself, had to call the Absa Credit Card department to check this wasn't a ****. The new card was automatically linked in my internet banking, however it never indicated how much I needed to pay on my due date. Instead it said: Due R 0 on 0000-00-00 and said I had a minimum payment that was less than R 300. I took a guess and payed R 4000 on the day my payment is usually due. Now I find out that they have charged me interest! How can they do this when they wouldn't even tell me what I needed to pay to avoid being charged interest! I demand that they reverse the finance charge as this cannot be legal.