Do you not see the irony in decrying new dams, then with the next breath advocating rainwater tanks? Why is one device for retaining rainfall bad, and the other good?

The issue here is one of scale and circumstance.
Placing a (large) dam in the catchment (if there are available sites) removes that water from use further downstream. This is one of the issues regarding environmental flows... eg the Snowy River. Secondly, what is the point of said large dam if it doesn't rain OR if the climate changes rendering that investment moot?

RE Rainwater tanks.
Given that 80 - 90 % of Australians are urbanites and mostly located in the big capitals... rain falling in our cities by and large just goes straight down the drain and out to sea. Streams and rivers receiving stormwater run-off are degraded from the flashy flows. The rain caught in tanks in urban environments can serve a dual purpose. Relieving stress on the drinking water supply by using rainwater for the garden and toilet (if you don't feel like drinking it) and potentially leading to partial rehabilitation of urban streams by catching a portion of the runoff thus reducing the intensity of flash flows (which also periodically drown people).

All the paving and hard surfaces in cities has changed the local hydrology. In Melbourne the extensive dry period has led to drying of soils (which receive less infiltration from rain) which has led to cracking of walls.