Daniel Morcombe's killer will remain behind bars for at least another 15 years after Australia's highest court refused his application to appeal his murder conviction.

Brett Peter Cowan has now exhausted all his legal avenues for appeal, after the High Court deemed there to be insufficient grounds to overturn the conviction.

Outside court on Friday morning, Daniel's mother Denise could not hold back her tears, as both her and husband Bruce expressed relief their countless years of court hearings in their fight for justice for their son were finally over.

"I'm just glad it's all over now and we can get on with our life," Mrs Morcombe said.