There are many factors that determine the frequency and length of any Grid Credit Event. Regions with higher Volatility Factors (VFR) historically tend to experience a greater magnitude of each. You can read more on the AEMO website.
In some years, high volatility events have been relatively frequent across both South Australia and Queensland coinciding with periods of high demand.
The number of high volatility events in New South Wales and Victoria have been increasing in more recent times.
How many Grid Credit Events will occur in the future? This is difficult to estimate. However, future closures of coal fired generators and higher gas prices are likely to have an impact.