A rare low so large it's likely to cover both Australia and New Zealand is expected this week, with heavy rain forecast to fall on both countries on Thursday.

New Zealand's northern coastline is expecting heavy rain around Northland, drifting towards Auckland and Coromandel Peninsula during the day.

In Australia the southeastern coast has been warned by Australia's Bureau of Meteorology about a very wet southeasterly starting on Wednesday and carrying on into Thursday.

Whilst the Australian coast will battle the cold, New Zealand can expect wet sub-tropical northerlies, because of the massive cyclonic flow.

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Later in the week MetService has "moderate confidence" of heavy rain for northern New Zealand.

For those in the South Island, apart from a wet easterly which may brush Canterbury and Marlborough on Friday, the majority can expect a large dry high pressure system after this.