Australia's National Archive is trying to establish how a letter flown out of Paris by balloon during the Prussian siege ended up in its collection.

The tiny letter was written by a man in the French capital in 1870 and is addressed to his mother in Normandy.

A (or 'Ch', Charles) Mesnier (or Mesmier) asks his "bonne mère" whether she and the family are safe.

The National Archives said there was no record of how the "intriguing piece of history" arrived in Australia.

Assistant director-general Louise Doyle told the BBC the letter emerged during discussions with its French counterpart about a potential exhibition.

"We were delving into our collections to see what material connects to France, and it was extraordinary to find this intriguing piece of history."

'Your devoted son'

Prussian forces besieged Paris from September 1870 until January 1871 - during the Franco-Prussian war - when the French surrendered the city.

The stamp shows the letter was posted in Paris on 7 December, in the middle of siege, and was received just over a week later by Madame Mesnier.

"We don't need any more than a note that you are in good health to reassure us, "writes the man, whose age is unknown, in small, neat script.

"So far the siege has not really had an effect on the state of our health. We don't have much meat every day and when we do get some it is not very much but we can easily get by as things are and no one in our household is complaining.

"Embrace for me uncle and aunt and Maria as I embrace you with all my heart. Your devoted son."

The man also details the passion and dedication his fellow Parisians displayed in holding back the Prussians.

"We cannot succeed in all our attacks but I have the firm conviction, my good mother, that the ultimate success will be for our just cause," he writes.

The tiny letter measures only 20cm x 13cm and is labelled "par ballon monte" - for delivery by hot air balloon.

Almost two million letters were sent this way during the siege, to bypass the enemy after cables were cut under the River Seine.

The balloons often carried important people west, as well as homing pigeons which would return messages back to Paris.

Letters were delicately folded into a tiny envelope so as many as possible could fit inside the balloon.

"I find it romantic and elegant all in the same breath, and quite a beautiful the relationship this man has with mother," said Doyle.

She said it was possible the letter was bought at auction and then donated to the Queensland Telegraph Museum, but its journey was not entirely clear.

The National Archives hopes to determine whether there is a solid Australian connection with the letter, and whether any of the sender's relatives are living in Brisbane.