Natural gas lines are very unlikely to explode on their own. Unless something extremely serious has gone wrong, they only contain methane, without oxygen, so even if they're cut and ignited all you'll get is a localized fire where the leak is.

Much more likely that natural gas leaked into an existing void containing air... such as a sewer line.

Methane explosion. If it was LPG gas there would have been a flame. Plus there are no gas lines in that particular block.

Given how incompetently utilities are run in South Africa, a gas leak in a municipal line would surprise me not at all.