I agree, the leaves are hard to see. It looks very much like something we have, Saponaria officinalis (L). Does the flower bloom ever so slightly pink in some years? Ours has a mild peppery/sweet fragrance, but not every year. It's also REALLY hard to kill. Common name, soapwort, Soaproot, Bouncing Bet, Latherwort, Fuller's Herb, Bruisewort, Crow Soap, Sweet Betty. http://altnature.com/gallery/soapwort.htm Good luck!
I grow S. officianalis both here on the coast of Sydneys northern beaches (sub tropical climate) and 80kms away in Leura (blue mountains) 1000m above sea level with a definite temperate climate and Ive never seen it grow with such an erect stem and growing so tall. I'm not sold on it being Saponaria I'm still backing Lychnis or maybe even phlox
Changed my mind after looking at some of the other alternatives - it doesn't seem to be a Lychnis, and my rationale for that is the images I've seen of Lychnis viscaria show a plant that either lacks or has smaller stem leaves - this image clearly has them, which makes me lean toward Ron B's assessment.
"A strong, spreading perennial, it is hairless or nearly so. The plant reaches 3' high. Stems are often purplish. Opposite leaves have 3 prominent veins, mostly no larger than 4 5/8" x 1 2/3"; they're soapy if rubbed between one's fingers and can be used for that purpose." -A.L. Jacobson, Wild Plants of Greater Seattle