Construction workers in Brisbane have unearthed animal and plant fossils believed to be about 50 million years old.

The fossils were contained in a layer of oil shale more than 15 metres underground at Geebung on Brisbane's northside.

The surprise discovery was brought to the surface by a piling rig last month during building excavation works.

Project engineers have been talking to experts from the Queensland Museum, who say the fossils are those of ancient crocodiles, fish, freshwater shells and plants.

Academics say the suburban site was completely unknown to them.

Professor Suzanne Miller, the chief of the Queensland Museum, says the find is not only unusual but significant.

"It's very unusual to find materials being uncovered in these urban sites. I mean, most discoveries of this nature are found miles from anywhere," she told The World Today.

"First of all it's very unusual to have an urban construction site that finds it, and secondly to have the people on the construction team realise that there was something unusual and to make that call was incredibly fortunate for us."

The fossils will now be subject to numerous studies. ( AAP: Dave Hunt )

Professor Miller says the museum now needs to find a site for the five tonnes of soil that road workers have set aside, and a team of community volunteers to help sift through the material.

She says the fossils could fill in a missing link in worldwide knowledge of what happened after the dinosaurs were wiped off the planet.

"We believe that there's real potential in this site to uncover something that will be absolutely unique for Queensland and really unique for Australia," she said.

"This is one of those sites that's showing us all these smaller animals that co-existed in one place in one habitat at the same time, just after the massive extinction that wiped out a vast amount of life on the planet.

"It's a really critical time for understanding how life then went on to evolve, but it's quite a bit before the time of the famous Australian megafauna where you have the giant kangaroos and so on.

"It's a bit of a missing link in the geological and fossil history, not just of Australia but within the world."

Brisbane Lord Mayor Graham Quirk says the find will not hinder the timetable for the project, although engineers are now liaising with researchers in case more unusual remains are unearthed.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Listen Duration: 4 minutes 31 seconds 4 m 31 s 50 million year old fossils found under Brisbane roadworks ( Nance Haxton ) Download 2.1 MB

"The project can continue, but the pylons have been drilled, and so it's really now a case of all of the spoil from those drillings, they will be taken away and examined carefully," he said.

"I understand the work will take two to three years to complete - there's that much spoil that has been drawn from this site.

"The project will continue - it's a $200 million road congestion and safety project.

"But certainly, this discovery is very significant.

"It could well have global consequences and significance, so we'll wait and see what that uncovers."