Supramental nature can only be understood if one understands what supermind is and that is not altogether possible for mind so long as it does not open into the higher planes. So far as a mental account can be given, I have done it in the Arya.
Divine Nature is the nature of the divine Consciousness, Truth, Peace, Light, Purity, Knowledge, Power, Ananda on whichever plane it manifests. Supermind is one plane of the Divine Nature. The Divine is Sachchidananda.
Ref: Letters on Himself and the Ashram