There are three very similar species of Stylidium in the ACT. Stylidium armeria subsp. armeria is widespread and has leaves 3-15 mm wide, the midvein on the back not obvious, and brighter flowers. Stylidium gramineum grows on sandstone eg Black Mountain and Bruce, has narrower leaves (to 3 mm wide) and a distinct raised midvein on the back of the leaves, which is difficult to photograph but easy to see and to comment on, and paler flowers. Stylidium montanum is at higher altitudes, grows in colonies, and likes wet feet.
Thank you for your descriptions on the different Stylidium species BettyDonWood, I'll make sure I get good shots of the leaves next time. This plant was on its own on a dry slope, so I figured it would be either armeria or gramineum.