While the sight of the D9 bulldozer in the river opposite the Duntroon golf course was a topic of amusement for passers-by, the construction union raised concerns about security on the site and said the culprit is probably lucky to be alive.

''It could be seen to be humorous and there's quite a few people ringing up laughing about it to me, but we can laugh about it now knowing that no one's dead,'' said Dean Hall, ACT branch secretary of the Construction, Forestry, Mining and Energy Union.

''That's a heavy piece of equipment. It could have quite easily rolled over the embankment, rolled in and gone underwater - and it would have been extremely hard for anyone to escape that.

''They could [have] drowned, [been] pinned underneath the water [or been] crushed by the cabin from the roll over.''

Mr Hall said construction site security included strict rules about how machinery is left.

''One of those things is all the keys are meant to be taken out of them and they're meant to be locked up,'' he said.

''You would assume that's happened - the result is you'd assume someone's very skillful and hot-wired it and driven it into the creek. If that's the case, someone who's too smart for themselves should have also learnt to drive it properly if they were going to try to steal it.

''Driving it into a river probably wasn't the best outcome.''

A spokesperson for Territory and Municipal Services said the Environment Protection Authority, police and WorkSafe ACT were investigating the incident.

Bunting had been put in the river around the bulldozer to prevent any potential spread of fuel.

''The contractor is working to safely remove the bulldozer from the river and remedy the situation,'' the Territory and Municipal Services spokesperson said.

''The incident is not anticipated to impact on the progression of the Majura Parkway project.''