My freezer recently warmed to -6 degrees Centigrade during a long power outage from its usual -18C. It's now on its super freeze cycle and cooling back down again. Is the food inside still safe to eat? -6 is obviously less than zero, the freezing point of water, but I've heard that food in a freezer can go bad even below 0. Is this true?
In your question, does "go bad" mean food quality or food safety?
As for safety, you were nowhere near the "danger zone" either temperature-wise or time-wise.
Initially safe, properly stored, frozen food that warms to -6C (or 22 F) for 30 minutes will not render it unsafe to eat.
In terms of food quality, freezers (most of which these days are self-defrosting), go through temperature cycles regularly. The problem with this is not food safety, but the accumulation of ice crystals in foods, which degrades quality. Freezer burn is the extreme example. Of course, attention to packaging (vacuum sealing, for example) helps.