It is possible to climb in the Grampians year-round. However, the prime bouldering season runs from April through September, with the best conditions usually found in the shoulders of this time when it’s usually both cool and dry. You can climb in summer, but it usually requires early starts or night sessions to avoid searing heat and mega-spooge. If you are climbing in summer beware bushfires are common – and for you Internationals especially note that on Total Fire Ban days it’s uncooked pasta and no coffee, you cannot even use a gas stove.
The mean minimum and maximum temperatures range from highs of 29.8°C during the summer months and lows of 3.7°C during the winter. Mostly, though, conditions are mild, although it can still get dishearteningly wet and cold.
The northern parts of the park are generally drier than the southern, and often the Victoria Range will be clagged in damp clouds whilst Stapylton will be dry and clear. Many of the steeper cave sectors – the Hollow Mountain Cave, Kindergarten Cave of Man Hands, for example – are fine to climb in all weather. If you are climbing after rain, note that sandstone becomes very brittle when soaked, and if you don’t give the boulders long enough to dry you run the risk of snapping holds and ruining it for everyone.
If the worst comes to the worst, try climbing at Mt Arapiles as its smaller size seems to attract the foul weather less.
The best site for checking the weather is Bureau of Meteorology.