Heavy clay is common across Melbourne's outer north, and on newer estates it's often compacted under a thin layer of topsoil. Clay holds nutrients well, but it drains slowly in winter and sets hard in summer. A bit of preparation makes garden planting far more successful.
Plant in autumn if you can.
Autumn is usually the best time for planting. The soil is still warm, winter rain helps roots establish, and plants have months to settle in before their first summer. Spring planting works too, but new plants will need regular watering through their first hot months.
Prepare the bed, not just the hole.
Avoid digging clay when it's wet, as it smears and compacts. When it's just moist, loosen the whole bed and mix in plenty of compost. Gypsum can help break up some clays, but not all; a simple soil test shows whether it's worth using. On poorly drained spots, building the bed up into a slight mound gives roots air in wet winters.
Dig planting holes about twice as wide as the pot but no deeper. Set the plant at the same level it sat in the pot, water it in well and finish with mulch kept clear of the stem.
Choose tough plants.
Plants that cope with frost, summer heat and clay will do far better than fussy ones. Plenty of Australian natives, such as lomandra, westringia, correa and many grevilleas, suit local conditions; kangaroo paws do best on a raised, well-drained mound. Councils and local indigenous nurseries publish lists of plants suited to the area.
Estate rules.
If your home is on a newer estate in Beveridge or Kalkallo, check whether the estate has front garden guidelines before you plan new garden beds.
Want a hand with it? See planting & garden beds in Wallan, Beveridge and Kalkallo.
Also worth reading: How to get an accurate garden maintenance quote, Regular gardener or one-off tidy-up: which makes more sense?.