A heartbroken dog owner has shared an emotional Facebook post following the death of her dog in a storm drain.

Madi McNaughton, 24, was out walking Rupert, a three-year-old mastiff mix, on a California beach, when the 'freak accident' happened.

"Today quickly turned into the worst day of my life and I am in complete disbelief... A freak accident took the life of my baby [Rupert the dog] today," she explained.

"I can barely stand to even share this right now but he was loved by so many. Rupert you were and always will be my soul dog and I can't imagine how my life will be without you.

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"At just three years old you had gone through so much and it's just not fair... I just want to wake up from this horrible nightmare that I'm in."

In a follow-up post she added: "I threw Rupert's ball for him, not realising that it would be the last time I ever got to play ball with my baby.... As he went out to get it a wave of high tide came in. Next thing I knew, I saw him getting swept towards the drain that you see in the video.

"Not even thinking, I dropped everything I had and jumped in trying to reach him... where the drain goes in was so deep ... I had no leverage and nothing to grab onto. I reached Rupert right as he was getting pulled into the drain, I had my one hand on his collar and the other gripping the wall above me trying my hardest to hold on to him but I couldn't do it..."

Her story has been shared by thousands of Facebook users.