Default interest rates are charged when a mortgage is in default. Consequently, all lenders will charge you a default interest rate on your home loan.
However, your loan contract will show how much you are charged for your default interest rate. Although, your interest rate would incur an extra mortgage arrears rate of between 3-5%. Hence, this default interest rate would be on top of your normal interest rate.
Indeed, some lenders will charge you a default rate on the arrears amount only. However, others will charge the default rate against the full loan amount owed. Hence, it is vital to make your payments on time. Consequently, a 4% interest rate could attract a 9% default interest rate.