Have not done it myself, but been told of two methods.
One is using very small baited hooks, the other is to head out at night with a fine meshed butterfly net and spotlight them. Then scoop them up, quite surprising how much more fish life there is after dark in many of our rivers.
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Whitebait Season starts 1 September 2026 and ends 30 October (inclusive). Chatham Islands: 1 December to the last day of February (inclusive)