It looks like Sonchus. Pity the photo of the leaves does not have the detail to make a definite ID. There are two common weedy species in the ACT. Were the leaves soft (oleraceus), or leathery with spiny edges (asper)? The native species has not been seen since about 1973. It has leathery leaves that are not spiny.
The lobes at the base of the leaves in oleraceus are pointed, which they appear to be in your photos. The lobes at the base of the leaves in asper are rounded.
Mike pointed out that this needed revisiting. On looking more closely, I could see that the bases of the leaves do not appear to be lobed. Vicflora says the lobes in asper are rounded and pressed close to the stem.