The water productivity of sunflower offers comparative advantages in water-limited semi-arid cropping systems. Deficit irrigation field trials were conducted to evaluate canopy and oilseed yield formation and water productivity. Deficit irrigation effects on canopy formation were determined by repeated measure of light transmittance (LiCor Plant Canopy Analyzer); above-ground biomass was determined by allometry; seed fill-rate, oil content and fatty acid composition were determined from bi-weekly samples; crop water use was determined by the soil water balance method using neutron thermalization. Maximum canopy coincided with the onset of anthesis in cool and normal growing seasons. In a cool season (2009) canopy was greatest for intermediate populations (57,000 pl ha^-1) relative to smaller (44,000 pl ha^-1) and larger (69,000 pl ha^-1) populations; no differences were detected in 2010. Seed fill rates (0.9 to 1.2 mg day^-1), maximum achene mass (37 - 45 mg), oil content (43.9% - 46.5%) and oilseed yields (3.2 - 3.6 Mg ha^-1) were favored by cooler growing conditions. Maximum achene mass was favored by increased irrigation capacity (2.0 - 6.4 mm da^-1) when no pre-season irrigation (127 mm) was applied. Oil content and oil yield increased with seeding rates in both growing seasons. Water use (440 - 510 mm) was reduced for the cooler growing season. Oilseed productivity is interpreted with respect to radiation use and photo-thermal quotient.