Amber electric 30min tarrif values

Has anyone noticed Amber Electric tarrif patterns like this?
As they charge for import and export on 30min increments, i’ve noticed a pattern where if grid price is negative, then it tends to go positive just before 30min increment roles over, so price for the whole just completed 30min is much higher than the average…

Red circles show the price charged for grid usage each half hour block.

Is this common with others experiences?
This snippet is from HomeAssistant (HA). I have a custom battery system, and control my import export via logic in HA.

Well done on documenting this! Very interesting. It might indicate that some third party is gaming the wholesale market – purchasing power at (say) 5 minutes after Amber Electric (and perhaps others) are selling the excess PV power which is queued up for being “dumped” onto the grid during the next half-hour. But a single day of apparently-clever timed trades won’t make a fortune for any arbitrager! It seems quite possible that the trading team at Amber Electric have already noticed this issue and are taking steps to stagger the timing of their wholesale purchases and sales? Is this pattern still occurring? Have you contacted Amber Electric to see what they have to say about it?

BTW I live in NZ. I have only the vaguest idea of how the wholesale electricity market in Oz works. Things are quite different in different jurisdictions!