A bumper year for the country's grape growers could be great news for wine buyers, but not vineyards.Good growing conditions mean that this year's harvest might exceed 300,000 tonnes, raising the possibility of another wine glut and a tough year for wineries.Oversupply has threatened to damage the value of New Zealand wines in key export markets since a bumper harvest in 2008. Grape prices subsequently slumped and, with them, profits.