I wouldn't take that theoretical LI value of -13.2C on that sounding too literally @Gleno71 - that figure would only be valid if Brisbane's forecast max temp was reached (fell a degree or two shy yesterday) and even if it was, it'd only be valid for that time. Brisbane Airport from where the sounding was performed typically tends to be a fair bit cooler than the city.wow so the Lifted index was -13??
Those particular soundings also rely on the assumption that parcels only rise from the surface. In reality on hot sunny days over land, parcels tend to rise from a well-mixed layer in the lower atmosphere, not just from the surface.
But you can see that even with the observed max temp of 30.5C in Brisbane and a mixed layer in the lower levels, the extremely steep lapse rates aloft mean that any parcels would be extremely unstable if they could make it up through that somewhat more stable layer in the lower levels. Of course it's better to also look at observed soundings a bit upstream of the location of interest, especially on a day like yesterday with such strong steering winds aloft advecting stuff into the area.... but you can see how generally unstable it was.