Predictions for July-Sept 4C (Solar production and receipts), page-123

Yes, that’s the rule. See Market Suspension Pricing Methodology here (important bit pasted below)https://aemo.com.au/en/energy-systems/electricity/national-electricity-market-nem/data-nem/market-management-system-mms-data/market-suspension-pricing-schedule:

“These schedules will be calculated and published each week and will be based on average values for each 30-minute period over the previous four completed weeks prior to the calculation. In calculating these values, allowance will be made for any changes to and from daylight saving time aligning values at local time.”

They just use the 30min price for each 5min interval, it does impact neighbouring regions also.

Although I think you missed a market notice, the suspension only lasted around an hour.
Market Notice 106784 - NSW declared suspended at 8:10 market time.
Market notice 106785 - NSW to resume at 9:10pm market time.
prices still remained > $100 for this period I think.