MetService has upgraded the chances of a tropical low near Fiji becoming a cyclone from moderate to high - and it could be a severe cyclone.
MetService is monitoring the low to the north of Fiji and on Saturday said the chance it could develop into a cyclone was 'moderate'. On Sunday it revised the risk of a cyclone forming in the next 24 to 36 hours to 'high', saying it "lies in a favourable environment".
"Potentially two tropical cyclones developing this week," MetService tweeted.
"At this stage, the risk of either directly impacting New Zealand is relatively low. However, there is a lot to happen before the tracks becomes certain so things could change."
It's expected to move west towards Vanuatu on Monday and Tuesday and the risk of it becoming a tropical cyclone while there remains high.
The system is expected to recurve south and move across or very close to Vanuatu during Tuesday and Wednesday, and is likely to reach severe tropical cyclone category 3 or above.
Another tropical low over the Coral Sea is expected to move east, and the chance for it developing into a tropical cyclone is expected to become low on Wednesday, moderate on Thursday and high in the outlook period. This system is expected to follow a similar track to the tropical low north of Fiji, and move close to Vanuatu late next week.
Weather warnings and watches
The South Island is now in the firing line for a band of heavy rain that brought fresh flooding and slips to parts of the North Island this weekend - but the north isn't out the woods yet, with heavy rain expected for some areas on Monday.
MetService said on Sunday morning the main rain band has now moved off the North Island and onto the top of the South Island.
An orange heavy rain warning has been issued for eastern Marlborough and northern Canterbury, with MetService warning heavy rain may cause streams to rise rapidly. Surface flooding and slips are also possible and driving conditions may be hazardous.
Eastern Marlborough south of Kekerengu and north Canterbury north of the Hurunui river is under a heavy rain warning from 7am until midday Sunday.
Gisborne and the Coromandel are both under a heavy rain watch for most of Monday and can expect periods of heavy rain with possible thunderstorms and localised downpours. Rainfall amounts may approach warning criteria.
The watch period for the Coromandel Peninsula is from 9am Monday until 3am Tuesday.
The watch period for Gisborne from Tolaga Bay northwards is from 3am Monday until 6am Tuesday.
Heavy showers and thunderstorms could also develop over Northland and west Auckland on Sunday afternoon.
A heavy rain watch that was issued for Wairarapa has now been lifted.
