An active front should move slowly onto southern South Island areas from Sunday afternoon, to lie across northern Fiordland and Otago at midnight on Sunday and then cross the rest of the South Island on Monday. A very strong moist northwest flow precedes the front and heavy rainfall warnings have been issued for western areas from northwest Nelson to Fiordland including the Canterbury Alps.

Following the front a cold south to southeast flow will bring widespread rain to eastern areas from Southland to Canterbury.

For these areas, rainfall totals between midday Sunday and midday Monday of about 40 to 60mm are expected. Although these amounts may well stay below warning thresholds, localised flooding is possible, especially for the hills and ranges of north Otago, south Canterbury and also for Christchurch city.

The watch for possible heavy rain in the northwest of Nelson has now been upgraded to a warning.

People in all these areas should be aware of substantial rainfalls coming and look out for any upgrade of these forecasts to a full warning.