Victoria's Department of Justice is investigating after a letter apparently written by jailed Cardinal George Pell to his supporters was posted to social media.

Key points: The Twitter page said Cardinal Pell sent the letter from prison

The Twitter page said Cardinal Pell sent the letter from prison It is a prison offence to commission other people to post to social media on prisoners' behalf

It is a prison offence to commission other people to post to social media on prisoners' behalf The department said it would "thoroughly investigate" the social media activity

Photos of the letter, which are dated August 1, 2019, were posted to the Cardinal George Pell Supporters Twitter account late on Friday evening.

"My faith in our God, like yours, is a source of strength," the letter says.

"The knowledge that my small suffering can be used for good purposes through being joined to Jesus' suffering gives me purpose and direction.

"Challenges and problems in Church life should be confronted in a similar spirit of faith."

The Twitter account said the letter was the "latest pastoral letter" from Cardinal Pell, who is serving a six-year jail sentence for sexually abusing two choirboys when he was the Catholic archbishop of Melbourne in the 1990s.

Cardinal Pell, 77, has maintained he is innocent and is awaiting judgement after appealing against the conviction.

The letter was posted late last night. ( Twitter )

Prisoners are allowed to write and receive letters from behind bars, but they are banned from encouraging others to post online on their behalf.

"Prisoners are denied any access to the internet or social media. It is a prison offence to commission other individuals to post something on social media on behalf of a prisoner," a department spokesperson said.

"The Department of Justice and Community Safety will thoroughly investigate this social media activity.

"Any prisoner found to be contravening prison regulations faces disciplinary action."

The ABC is not suggesting Cardinal Pell encouraged the group to post the letter and has not verified its authenticity.

The Twitter post showed two pages of the handwritten letter, which said it was sent from the Melbourne Assessment Prison.

The letter thanked members of the Support Cardinal Pell group for their prayers and offered "a word of explanation" for an apparent delay in replying, saying he had received between 1,500 and 2,000 letters.

The letter urged the group to "always remember that the Catholic Church is one".

Cardinal Pell's lawyer Paul Galbally declined to comment when contacted by the ABC.

Editor's note: On Tuesday April 7, 2020, the High Court in a unanimous decision upheld Cardinal Pell's appeal and quashed his convictions on all five charges.