Octopuses are hurling silt, shells, and algae at each other, but scientists are yet to determine whether they are trying to brawl or just attempting to maintain some personal space.
Underwater cameras set up in Jervis Bay off NSW's south coast filmed about 10 common Sydney octopuses for more than 20 hours, and at least half of them launched "throws".
Females were seen propelling objects significantly more than males, with one repeatedly flinging silt at a male after he kept trying to mate with her. He had to frequently duck to avoid her hits.
University of Sydney Professor Peter Godfrey-Smith clarifies that the animals aren't throwing in the human-sense - instead, they gather material up in their arms and hold it in their arm web.
They propel whatever they're holding forward, sometimes several body-lengths away, by using their siphon, which is a funnel next to their head.
"In some cases, the target octopus raised an arm up between itself and the thrower, just before the throw, perhaps in recognition of the imminent act," Prof Godfrey-Smith, who is the study's lead author, said.
The researchers have dubbed the area where they analysed the animals in 2015 and 2016 as "Octopolis", given its especially high density of octopuses and cramped conditions.
Whether the throws they saw are acts of aggression, they can't be sure - mainly because they haven't seen "return fire" to prove it, and most throws do not hit other animals.
Throws that appear aggressive could be because of the cramped conditions, Prof Godfrey-Smith suggested.
"Only a minority of cases appear to be targeted," he said.
"I'd speculate that a lot of the targeted throws are more like an attempt to establish some 'personal space', but this is a speculation, it's very hard to know what their goals might be."
The researchers also saw octopuses throwing the remains of meals out to clean their dens, and hitting fish with their throws.
In one case, an octopus landed a hit on one of the cameras.
Despite the lack of evidence about the octopuses' motivations for hitting each other, researchers did find a correlation between the animals' colour and the energy behind their throws.
Earlier work at the site found octopuses with darker hues were more aggressive.
"Octopuses that displayed uniform colour (dark or medium) threw significantly more often with high vigour, while those displaying a 'pale and dark eyes' pattern threw more often with low vigour," Prof Godfrey-Smith said.
"Throws by octopuses displaying uniform body patterns (especially uniform dark patterns) hit other octopuses significantly more often than in other body patterns."
Octopuses more often hit each other with silt rather than shells or algae.
The research is being published on Thursday in the PLOS ONE journal.