The corresponding graph is very interesting as well [0].
Demand rose steadily from 4:30am through to its peak at 4:30pm. Just after this the spot-price jumped from ~$100 where it had been fairly steadily throughout the day to ~$2300. The market quickly corrected itself, but that half an hour would have been expensive.
Assuming an average price of $2350 per unit, for 2800 units, that half an hour would have cost around $3.43M. In all likelihood the market would have corrected much more quickly than that, but even at half that price it is still a lot of money.
[Edit]
The market is actually calculated in 5 minute intervals, and this graph [1] shows it broken down like that. From it we can see that for 5 minutes the load was at 3050 units, and the price was $12750. The price is an hourly rate, so if this had continued for the hour it would have cost $38.9M. As it was for only 5 minutes, it was actually 'only' $3.24M
Looking at the half-hour averages, this seems to match up, so maybe we were fairly close to begin with.
NSW1 $55 10549.74 QLD1 $80.99 6991.47 SA1 $2356.86 2761.4 TAS1 $38.99 1206.17 VIC1 $47.7 7995.27
Look at the value for SA1, this may be because SA is going through a heat wave, where temperatures have reached 47.4 degree over the last few days.