Soft under the skin in Neobuxbaumia

I noticed on my N. polylopha the other day that the cortex is soft all around the plant just up from the base but the rest of the plant is fine. I saw no holes or obvious rot anywhere and the plant is still growing and has healthy roots. It is not related to the rains because I noticed it before then.
The whole plant is only ~25 cm tall, from seed and the base is woody. The wood is fine because it can transport water and does not bend.
It doesn't look sick and it does not feel as bad (but still pretty bad) today as it was yesterday.

Anyone know what this problem might be?

Its hard to say with out seeing it, any chance of a photo??
i would stop watering it at all, remove from pot and see what the roots are doing, i have had a few plants go bad and found that the roots were all rotten.

I had a similar thing happen to one of my small plants (unidentified species sorry). It went 'squishy' feeling as though it had rot, so I cleaned the roots off and they were healthy. No signs of what might have caused it so I repotted it and pampered it for ages and it's now recovered and growing happily.
I figure it must have been too cold one night and gotten mildly frosted since it had been inside and it was put out early in the growing season, because it was so small it must have been more susceptible to the cold nights. I'm happily amazed that it's recovered well.
I wouldn't imagine cold to be a problem in Sydney at the moment but with the hot/ cool spells that're going about ... maybe. Hope yours comes back too.

It has been out in full sun (and recently, full rain...) since the middle-end of winter so maybe it got too hot? I noticed it on a really hot day.

Pictures probably won't help (but I might post some anyway) because it looks normal and unharmed. The only difference is the skin looks a bit buckled under its weight because the cortex isn't holding it stiff.

Now the softness has disappeared almost entirely and the plant appears to be thickening at the bottom with more secondary growth. I will continue observations to see if anything happens.