1kg pack of Green Manure Seed will cover about 20 square metres.  The pack consists of a mixture at least a dozen types of seeds (which can vary seasonally) but may include:

cereal rye, lupin, pea, canola, sunflower, safflower, panicum, triticale, oats, buckwheat, millet, canola, mung bean, sorghum, alfalfa and brassica seeds.

Depending on the season and local conditions, some seed varieties will do better than others; but the idea is to get a broad range of actively growing crops up.  Green manure should be allowed to grow, but before seeds set, slash and dig into the soil.  This adds valuable organic matter to the soil as it breaks down, and roots help break up heavy or compacted soil, improving aeration and conditions for beneficial microbes.  Brassica crops also help eliminate root knot nematode.