In a now-deleted tweet, a French company might have revealed Australia's closely held submarine maintenance plans, suggesting work could soon be a thing of the past in Adelaide.

Key points: Naval Group tweeted images of a private briefing with French Senators visiting Adelaide last month

Naval Group tweeted images of a private briefing with French Senators visiting Adelaide last month One of the photos appeared to show a map for a proposed submarine construction yard site in SA

One of the photos appeared to show a map for a proposed submarine construction yard site in SA The Federal Government has maintained no decision has been made as to where Attack-class boats would be built

Naval Group, who were chosen in 2016 to build Australia's future submarines, gave a private briefing on the $50 billion program to a group of French Senators visiting Adelaide last month.

To mark the occasion, Naval Group Australia — the local subsidiary of France's Naval Group — tweeted a series of photographs that included a partial glimpse of a map showing the proposed site for the submarine construction yard ("SCY").

The now-deleted image appeared to confirm planning was underway to build the new submarines at Adelaide's Osborne North precinct, meaning maintenance work on the current Collins Class fleet would likely have to move to Western Australia.

However, Defence and the Federal Government have consistently argued that no decision has yet been made on precisely where the Attack-class boats will be built, or where "full cycle docking" for the Collins Fleet will occur after 2024.

"To avoid misinformation on the future submarine yard, Naval Group Australia decided to remove the image from its social media sites," the Defence Department told the ABC in a statement.

Maintenance for Collins class submarines is performed in South Australia. ( ADF )

Just last week, a document released under freedom of information also revealed that in 2017 the Royal Australian Navy had instructed Naval Group Australia to begin planning its submarine construction at the Osborne shipyards on the assumption Collins Class maintenance would move from Adelaide.

"As previously stated by the Minister for Defence, no decision has been made by the Government on the future submarine yard or full cycle docking," the Defence Department insisted in its statement.

"Options for future submarine support will be presented to Government for consideration at the end of this year."

A tweet from Naval Group featured multiple images from the meetings with the French Senators. ( Twitter: @navalgroup_AU )

South Australian crossbench Senator Rex Patrick has now accused the Australian Navy of "not being forthright with information to the Australian Senate".

"It appears the French Senate gets a full briefing while the Australian Senate is kept in the dark," Senator Patrick said.

"The only reason I have been able to keep up with what is going on is because I have been extracting the story bit by bit using freedom of information laws. This situation will have to change".

The Defence Department insists "no sensitive information was provided to the visiting delegation" of French Senators during last month's visit to Adelaide.

"Naval Group Australia provided the delegation with its briefing on the progress of its work on the Attack Class Submarine Program including options by Naval Group for development of the submarine construction yard," the Department said.

Former defence minister Christopher Pyne was a vocal supporter of South Australia's military maintenance work.

His ministerial successor, Senator Linda Reynolds, and Defence Industry Minister Melissa Price are both from Western Australia where the State Labor government is campaigning vocally to take over the lucrative maintenance work for Collins Class submarines.

Naval Group Australia has been approached for comment.