An Indigenous-Australian artist is pleading with Australia Post for help after her irreplaceable painting worth $4000 became lost in transit over Christmas.

Bobbi Lockyer spent more than 100 hours painting a one-of-a-kind commissioned artwork for a customer and, like many of her pieces, sent it via express post.

Lockyer, who is based in Darwin, told nine.com.au she was instructed to pack her tube in a cardboard sleeve before paying the postage and sending it away.

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On Christmas Eve, Lockyer's customer sent a frantic message.

The only thing that had arrived on her doorstep was the empty cardboard sleeve.

"She sends me a picture of the empty cardboard sleeve and my heart just dropped," Lockyer explained.

"I immediately drove down to the post office on Christmas Eve, it was so busy and I waited in line."

Lockyer said she was told Australia Post would open up a case and trace the package route to find where the tube containing the painting became lost.

For the past several weeks, it's been an agonising waiting game.

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