Wetland Weed ID

I saw this recently, growing in damp area at the bottom of a small gully in Christchurch, New Zealand. The plant appears to be thriving, so I'm guessing it must have a propensity for growing in wetland. The flower has the look of a lobelia about it (though some of the flowers appear to have 6 petals) and the red stems are quite distinctive. The height is 20-30cm and the flowers are no more than 6-8mm diameter. Any ideas?

Thanks

 

Looks like Lythrum hyssopifolia,

 

Wonderful. Thank you very much, that's certainly what it is.