A number of factors affect the daily count, including daily fluctuations in temperature, wind conditions, humidity and rainfall, and of course the biology of the plants themselves.
Most grasses flower in late Spring and early Summer. During the flowering season, weather conditions such as wind and humidity will also affect how much grass pollen is in the air. In the Sydney region peak grass pollen counts occur in October/November with some year to year variation. In most years there is another, lower peak in the summer.