Dairy cattle in New Zealand

Farm: For the purposes of this census a farm is one or more blocks of land, managed as a single operation, that is engaged in agricultural activity. This includes farming of livestock, horticulture, viticulture, nurseries, forestry, growing grain and seed crops, and land that could be used for these purposes.

Bull: entire (ie not castrated) male cattle.
Calf: young cattle of either sex between birth and weaning, with eight temporary teeth. Also sometimes refers to animals up to one year old.
Cow: mature female cattle beast of any age, but usually over 30 months.
Heifer: female cattle having no more than six permanent incisors.

Counts as at 30 June 2012.

http://www.stats.govt.nz/browse_for_stats/industry_sectors/agriculture-horticulture-forestry/AgriculturalProduction_final_HOTPJun12final/Data%20Quality.aspx

The compulsory registration level for GST is $60,000 so there is a partial and unquantifiable coverage of units below this level.

The Agricultural Production Surveys and Census include all units identified on Statistics New Zealand's Business Frame as having agricultural activity. The Business Frame is a list of businesses in New Zealand, based on their registration for goods and services tax (GST) with Inland Revenue. In the case of agriculture the Business Frame includes the following units: - those units registered for GST (and filing a non-zero GST return) that have indicated involvement in agricultural activity (as defined above) - those businesses with more than $200,000 of GST sales or purchases, or that are an employer, whose main activity is not agriculture but have indicated some secondary agricultural activity.

At URL provided, select 'Dairy cattle numbers, by age, sex and region, at 30 June 2012'.