The survey followed confirmation that a parliamentary select committee set up before the election to scrutinise how the price of milk is set will continue its inquiry.
The select committee has been looking into the price of milk at several levels, from farmgate to the supermarket chiller, but it was not known whether the committee would continue it after last year's general election.
The Dairy Workers' Union reportedly welcomed the committee's decision to continue, saying "the real pressure points on milk and dairy product pricing need to be exposed".
According to Statistics New Zealand, the average price of a standard two litres of milk in December 2011 was $3.67, up 1.7per cent on December the previous year but 15.4 per cent higher than in the same month in 2009.
The committee, headed by Lianne Dalziel in 2011, is now headed by National MP Todd McClay.
Public outcry has seen some retailers lowering the prices but the two major supermarket chains have refused to follow suit.
Foodstuffs and Progressive Enterprises argue their focus is on giving customers best deals on all products and not just a selected few.
Two-litre milk prices in Whangarei:
Supermarkets: Pak'n Save, Anchor $4.20, Budget $3.50.
Countdown Okara: Anchor $4.74, Homebrand $3.54.
New World Regent: Pams $3.89, Cow and Gate $3.59.
Service stations: BP Riverside, Anchor $5.20, Dairy Dale $3.99. Shell Water St, Meadow Fresh $5.59. Mobil Walton Plaza, Anchor $5.49, Dairy Dale $3.80.
Dairies: Daffodil Maunu Rd, Cow and Gate $3.99 or two for $6.80. Kensington Food Mart, Meadow Fresh $4.50.