Australian opener David Warner has made an impassioned appeal for the return of a rucksack containing his baggy green cap that went missing on a flight from Melbourne to Sydney for his final Test match this week.

The 37-year-old will bring down the curtain on his 12-year career in the longest format of the game in the third test against Pakistan at his home Sydney Cricket Ground starting on Wednesday.

"This is my last resort, but my backpack, which contained my Baggy Green, has been taken from my luggage, which was transported to Melbourne airport and flown ... to Sydney a few days ago," he said in a post on Instagram.

"It's sentimental to me, it's something that I would love to have back, in my hands, walking out there come this week.

"If it's the backpack that you really wanted, I have a spare one here. You won't get into trouble, just get in touch with Cricket Australia or me. I'm happy to give this to you if you return my Baggy Green."

Australian internationals receive the baggy green cap, usually from a former player, on the morning of their test debut and wear it with pride even as it deteriorates over the length of their career.

The baggy green cap of Australian cricketing great Shane Warne sold for A$1,007,500 ($686,913.50) at an auction in January 2020.