Amid the sea of coronavirus statistics, and an exponential rise in cases in NSW, there's one number that has experts particularly worried.

Between March 14 and 22, the total COVID-19 infections in Australia's most populous state went up from 112 to 669, including an increase of 276 cases where transmission occurred overseas.

But during that period, the number of cases in which the source is unknown rose from 11 to 65 — a 491 per cent increase.

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