SYDNEY (AFP) - Australian builders doing roadworks have uncovered a rare urban trove of crocodile and other fossils thought to be around 50 million years old, officials said Tuesday.

The fossils, trapped in a layer of oil shale, were found during excavation works near Brisbane's Geebung railway station at a depth of about 15 metres, according to city mayor Graham Quirk.

"The bones have been identified as from ancient crocodiles, as well as other significant material including fish, freshwater shells and plant impressions," said Mr Quirk.

Geoscientists were called in to examine the find, which Queensland Museum chief executive Suzanne Miller described as "particularly significant".

"Very few sites of this age are available for study, as similar-aged sites in the greater Brisbane area are often no longer accessible due to housing and urban development," Mr Miller said.

"The construction works have fortuitously provided access to a new locality that was not previously known to palaeontologists."

Queensland Transport Minister Scott Emerson said it was an "extraordinary finding in Brisbane's backyard" and the area would be combed for further specimens.

"The fossils will provide a valuable opportunity for more detailed studies," Mr Emerson said, adding that experts were interested in retrieving any other fossils that could be in the same vicinity.

Queensland has some of Australia's richest fossil deposits, including a famous dinosaur dig at Winton in the state's west, where three new dinosaur species were discovered in 2009.

Brisbane is Australia's third largest city, with a population of 2.2 million.