Combination of natural and man made causes. Onshore northerly winds pushing off shore reef weed inshore combined with councils flushing weed out of estuaries (at times badly as happened in the Clive river) which then spreads onto beaches. It should be extracted from the waterway completely but they take the cheapest option.
Its interesting that at a local lake care group I belong to an environmentally nontoxic weed killer was referred to and should be used on a wider scale to killoff weed in waterways which is normally caused due to artifically high nitrogen runoff from dairy conversions.