Shares in top stock Telecom fell 9c in early trading to 265 following announcements on mobile regulation by the Commerce Commission.
Shortly before the market opened the commission said it was to investigate whether the price of national mobile roaming should be regulated.
It also said that in a draft report it had recommended mobile termination prices be regulated.
Telecom's fall helped pull the whole market down, with the benchmark NZSX-50 index down 4.85 points around 10.15am to 2770.27, erasing yesterday's modest 4.5-point gain.
Pumpkin Patch shares rose 16c to 151, after the company said it was to close 20 of its 35 stores in the United States.
Other shares rising early included Auckland Airport, up 3c to 161, Sky City up 3c to 267, and The Warehouse up 3c to 375.
Shares up 2c included Freightways to 285, Mainfreight to 427, NZ Oil & Gas to 158, Nuplex to 150, Restaurant Brands to 104, and Sky TV to 424.
Early falls included Sanford down 10c to 540, Trustpower down 5c to 765 and Tower down 4c to 176.
United States stocks rose as higher oil prices lifted shares of energy companies and fund managers snapped up this quarter's winners to burnish their portfolios.
Fund managers are enhancing their portfolios before the quarter ends tomorrow in a ritual known as "window dressing" by selling some of the quarter's losers and scooping up the winners.
The Dow Jones industrial average gained 1.1% to end at 8529.38, the Standard & Poor's 500 Index was up 0.9% at 927.23, and the Nasdaq Composite Index was up 0.3% at 1844.06.
