In December, the Bureau of Statistics priced a two-litre bottle at $3.61. By March it was $3.52, and by June $3.41.

The bureau cautions the list of average prices it released yesterday can be misleading where product quality or size changes.

But neither the quality nor the quantity in two-litre milk bottles has changed, while the price of other dairy products such as butter and cheese has kept climbing.

Late last year Parliament abolished a tax that added 11 cents a litre to the price since the start of the decade.

Called a levy rather than a tax when introduced by the former prime minister John Howard, it applied only to the price of drinking or ‘‘pouring’’ milk and was designed to slowly raise the $2 billion paid to help dairy farmers adjust to deregulation.

By February, it had reached the total and the legislation to remove it took effect.

Sydneysiders have received almost all of the 11 cents benefit, but not Melbourne residents – who are paying just 4 cents a litre less. Perth residents are paying 16 cents a litre less.

The cut is one of the rare cases where Labor has used legislation to deliver on its promise to take action on grocery prices.

In another price cut, it has had help from Cadbury. In the past three months the recorded price of a large block of milk chocolate has slipped from $4.58 to $3.86.

Until March the bureau priced 250 gram blocks. In June it changed to 200g blocks. In April Cadbury cut the size of its 250g block to 200g and cut the wholesale price of the block.

Consumers saw a packet that looked about as big (thanks to some clever cardboard packaging) but paid less for it.

‘‘It’s a move towards responsible consumption,’’ Trish Hyde, chief executive of the Confectionery Manufacturers Association, argues, pointing out that Nestle and Mars have also cut the size of their bars.

‘‘The entire industry is doing it voluntarily. We are reverse-engineering our portion sizes in order to better align them with how much it is responsible to eat.’’

The move also helps the chocolate makers, with the price of sugar soaring in the past two years and the price of cocoa doubling.