A dog in Melbourne, out with its owner for an early morning walk, has led to an unexpected find in a dumpster — a vivid colour set of design plans for South Australia's Parliament House.

Key points: The designs will be gently restored and preserved after suffering some damage

The designs will be gently restored and preserved after suffering some damage Construction of South Australian Parliament started in the 1870s but was not completed until the 1930s

Construction of South Australian Parliament started in the 1870s but was not completed until the 1930s Six of a possible 48 design plans were found in the Melbourne bin

The head of the South Australian Parliament Research Library, Dr John Weste, said while he was used to historic materials surfacing in odd spots, the dog's find was one of the more unusual discovery tales he has encountered.

"He [the owner] was taking the family dog for a walk through Melbourne at about 6:00am, and came across a few cylindrical metal tubes in a skip," he said.

"He thought they might be interesting to have, and he took them home. When he opened the cylinders, he found these designs in them.

"It was complete luck."

Dr Weste had no idea the plans even existed until a few weeks ago when the mystery Melbourne dog walker and his wife rang and offered to donate them.

"I said 'yes please, I'd like them very much!'" Dr Weste said.

Dumped design plans are the 'fabric of the building'

Just how plans for the South Australian Parliament ended up being dumped in a Victorian skip bin is not clear and, in all likelihood, never will be.

The set of coloured design plans was found in a Melbourne bin. ( ABC News: Simon Royal )

But what is clear is how important the six separate sheets of drawings and specifications are to understanding the structure and history of SA's Parliament House.

"Until these designs were donated a couple of weeks ago, we had just one black and white copy printed on linen," Dr Weste said.

"They are literally the fabric of the building in the sense of how it was built, how it was constructed, down to the very details of the materials used, how the various rooms were originally configured.

"They are an insight we didn't have before.

"They are hand-coloured with watercolours, hand-drawn in intricate detail, they are pieces of art as much as anything else."

Conservator Anthony Zammit said the tin cylinders in which the plans were stored have proved to be a mixed blessing — they have preserved the vivid colours used by the designers, but they have also caused damage.

"Most of the damage that's been done is because the plans were smaller than the cylinders," he said.

"They've been able to move around and the edges have been severely damaged. They are brittle."

South Australia's Parliament House as its stands now on North Terrace. ( ABC News: Eugene Boisvert )

Brittle plans will be restored and preserved

Mr Zammit will now oversee a process of gentle restoration and preservation.

The tin cylinders will be consigned to the dustbin of history — at least as a storage option.

"No, no, no, they shouldn't be rolled up again. They must be stored flat," Mr Zammit said.

Construction of SA Parliament took several decades. ( SA Library )

The plans, which date from the 1930s, also highlight a curious, reoccurring quirk of South Australian history — the state's tendency to run out of cash at inopportune moments.

Construction of the magnificent Kapunda marble Parliament building actually began in the 1870s, but there was not enough money to finish the job until 1939.

Heritage architect Andrew Klenke said the result was that, for half a century, SA literally had half a Parliament.

"Everyone at the time thought it would be about 10 or 15 years before the building was finished. As it turned out, it was 50 years," he said.

"It's why the building has a Victorian exterior, but in the new parts there's more of an art deco feel to the interior."

More plans still missing

One of the designs for SA Parliament that was found in Melbourne. ( ABC News: Simon Royal )

But while the six plans are a more extensive set of records than Parliament's ever had before, they are far from complete.

Dr Weste said there should be 48 of them in total, all in the same gorgeous, vibrant colours.

Sadly, the library chief is not counting on another "man walking his dog" tale of good luck.

"It does make you think, 'What else gets thrown into a skip but isn't noticed by a man walking his dog at six o'clock in the morning?'" he said.

"I just have to be grateful for what we've got, we are very fortunate indeed."