Occurrences of cyanobacterial abundance and their consequence in toxin produced by the bloom-forming cyanobacterium Microcystis aeruginosa were studied based on the data collected from 2003 to 2005 in the Wingecarribee reservoir, New South Wales, Australia. Environmental conditions associated with cyanobacterial abundance were high pH, low dissolved oxygen, low light penetration, high total nitrogen, and low nitrate/nitrite-nitrogen and ammonium-nitrogen (p-values < 0.05, r² = 0.80). Total nitrogen was found to be a nutrient limiting for cyanobacterial abundance in this reservoir rather than total phosphorous. The toxic bloom-forming cyanobacterium M. aeruginosa was most dominant in summer 2004 and autumn 2005 when a ratio of total nitrogen to total phosphorous was high (95%CI: 19.81--23.96) due to low total phosphorous concentration (95%CI: 18--24 μg per litre). However, microcystin concentration was detected only in summer 2004 despite high toxic bloom-forming cyanobacterium M. aeruginosa at the time. Our study suggested that the formation of toxic or non-toxic cyanobacterial blooms in the reservoir was associated more with nitrogen than with phosphorous and toxic cyanobacterial bloom was influenced by high ratio of total nitrogen to total phosphorous and ammonium-nitrogen. Microcystin concentration may not be a good indicator for M. aeruginosa dominance.