So it's likely that it's suffering a bit of sun stroke. Best to keep it in a bright but sheltered spot while it recovers.
Also 3 waters (2 from you & the rain) in 3 weeks is quite a lot.
QLD weather is obviously very different to VIC but still need to ge mindful of not drowning the plant, it's much easier to kill a cacti from too much water than not enough (I've killed plenty this way!)
See how the little guy goes in the sheltered spot, should bounce back in a few weeks.
If the plant body becomes soft it could be rot which aside from grafting any remaining healthy tissue is not fixable.