The flower looks like the species suggested but the leaves are certainly wrong. I wonder if another plant is growing over the top of this species. There are one or two leaves underneath that look like Ranunculus.
I will check the leaves in a few days time. There could have been some weeds growing up through the buttercups, but I could not get very close to the plants as they are mostly underwater because the wetland has expanded with all the rain.
I have checked these plants again and added more photos. There were no weeds growing among the plants. As you can see from the new photos, there are normal leaves growing alongside abnormal leaves from the same plant. The leaves that have been mostly underwater show the most abnormal forms. As almost all the plants were underwater for a couple of weeks, this may have caused the unusual leaf form.
VicFlora states about the leaves 'lamina c. circular in outline, 1–5 cm diam., glabrous, deeply 3–5-lobed with entire or notched, ovate to cuneate segments (emergent form) varying to palmatisect with linear segments 1–3 mm wide (submerged form).'