The MetService said a warm afternoon and evening on top of light winds would likely lead to cloud buildups which could bring showers or thunderstorms about the southeast of the South Island.
For eastern and parts of central Otago and Southland there was a moderate risk of thunderstorms this afternoon and evening which could bring heavy rain of 10mm to 25mm an hour and hail.
Similar conditions were expected tomorrow afternoon and evening, with thunderstorms also possible in a broader area of the southeast of the South Island and parts of Canterbury.
There was also a chance of thunderstorms in parts of Fiordland and southern Westland.
The MetService warned heavy rain could cause streams and rivers to rise rapidly. Surface flooding and slips were also possible and driving conditions could be hazardous.
Total rain accumulation for the event could reach above 500mm.
A heavy rain watch was in place for Fiordland north of Doubtful Sound from 1am tomorrow to 1pm Sunday.