I had often seen them, or more accurately _heard_ their alarm calls as they became agitated.
Recently though, I have twice seen them do very much more than just "warn".
I have seen them actively threaten several different species of birds so vigorously that they retreated.
The first time was in early March at a water tray / bird bath, on a warm day.
The new (to me) squawking* sound draw my attention from inside the house, though the windows were open, so that is not too surprising.
More unusual and surprising was the obviously aggressive stance; head and tail down and feathers of the back raised, such that its profile was larger than usual.
The first incident began with five New Hollands bathing and squabbling among themselves and chasing other birds away. having all chased another away, just one returned alone, to find two wrens bathing.
The female wren took off immediately but the male hopped onto the side of the dish and stood his ground.
The seemingly bemused New Holland Honey Eater, was hassled energetically enough by the Blue Wren to be driven it off.
I could almost see it thinking, "Please don't anyone see this ... it's supposed to be the other way around .. someone tell it! Oh the shame!"
The wren began from the opposite edge of the bowl, then gradually shuffled closer and closer, squawking all the time, feathers fluffed out and beak wide open.
Once the Honey Eater had been driven off, the other blue wren returned and continued bathing while th aggressive one stood on the edge; after which it took the plunge.
At that stage I thought it likely that the young Honey Eater was more readily frightened than a larger or more mature bird, however later I saw the wren (the same one?) see off a Yellow Faced Honey Eater .
Later again a White-fronted Treecreeper approached the birdbath by creeping up the trunk towards the bowl.
It is quite awkward and takes a while to "make it" to the rim.
Two wrens this time, both squawking loudly and fluffed up threateningly crowded it until it flew away.
I'll let the pictures tell the rest of this story.
All's Well :)
* the second, informal definition fits perfectly.