They may look like one of dad's incomplete DIY pizza ovens, but the concrete bunkers being hefted into the Molonglo River hold more potential than a backyard thin crust.
They are in fact artificial fish habitat structures and 50 of them are being placed in the river to encourage vulnerable Murray cod to breed.
Project manager of the restoration of waterways project, Rebecca Dawson, said this would be a long-term environment restoration.
''It will take a lot of commitment, but it's a small part in giving something back,'' she said.
The structures, called reef balls, are a trial design and the first of their kind in the ACT and they're small enough to not intrude upon the river's jet ski laneways.
Aquatic ecologist Matt Beitzel said the artificial habitats would help preserve Murray cod.
''Murray cod prefer to breed in areas with flowing water, rather than lakes. The cod do breed in Lake Burley Griffin but not as successfully,'' Mr Beitzel said.
The idea to introduce clusters of reef balls into the Molonglo River was inspired by a similar project in Botany Bay.
''But we couldn't install something like concrete logs because they take up far too much space,'' Mr Beitzel said.
''So we found that the reef balls were more suited to the depth of water in the river.''
The main issues threatening Murray cod in the ACT include flow diversion from dams and invasive fish species such as the redfin and carp. Colder water released from dams can interfere with the temperature in which Murray cod prefer to breed, which varies from 16 to 20 degrees.
In February this year, more than 37,000 Murray cod fingerlings were released into Lake Burley Griffin by the National Capital Authority and the Canberra Fisherman's Club.
Ms Dawson said this project was a combined effort and required conservation groups to manage the rivers throughout the ACT and NSW. ''As the rivers go through both NSW and the ACT, we need to work together on this,'' she said. ''Queanbeyan Council is also removing willows, as they pose a threat to the waterways.''
Evidence of re-vegetation can be seen along the banks of the Molonglo, where native trees have been planted. But it will take almost 30 years until the trees have an effect upon the river and can provide a natural habitat for the fish.