Flood watch as downpours tipped to mix with king tides

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The North Island was bracing for flooding last night as king tides were expected to combine with heavy rain.

MetService warned of likely slips and surface flooding as a result of intense rainfall - in some places up to 50mm in an hour.

It issued a heavy rain warning for Auckland and Hawkes Bay and forecast up to 200mm would fall by this morning in Coromandel, the Bay of Plenty and Gisborne.

The "super moon" observed on Sunday evening was expected to draw tides to their extremes, and threatened to flood low-lying coastal areas on the east coast of the North Island.

The most at-risk areas overnight were coastal communities in Coromandel, Bay of Plenty and Gisborne, where downfalls were forecast to coincide with the high tide.

MetService weather ambassador Bob McDavitt said the most threatening high tides in Auckland would occur today, about 10am and 11pm.

The tidal range - the difference in height between low and high tide - is greatest in the days after the moon reaches its perigee, or the closest point of its orbit to earth.

King tides alone do not always cause problems for coastal areas. But other factors such as onshore winds, low pressure, or heavy rain at high tide can amplify the tide's size or strength.

Lower atmospheric pressure "sucks" the water level upwards. For every hectoPascal that the air pressure drops below the norm (1013 hPa), the sea level rises another centimetre. Air pressure was last night measured at 1000 hectoPascals.

On January 23, when a storm surge inundated coastal areas, the air pressure was as low as 980 hPa.

The combination of air pressure, northerly winds, and a king tide caused the largest tide on record in New Zealand, 4.67m in Onehunga.

Last night the high tide was forecast to reach about 3.6m.

The Bay of Plenty Regional Council advised farmers to move stock to high ground and said people should avoid travel unless necessary.

MetService predicted between 100 and 150mm of rain about the Bay of Plenty ranges in the 15 hours starting about 9 last night, with 70 to 100mm possible elsewhere in the region. Rains could reach between 25mm and 40mm an hour this morning.

Thames-Coromandel District Council civil defence manager Ron White said rain in Coromandel would probably be intense, with 120 to 180mm expected to fall about the ranges, and 70 to 100mm elsewhere in the region.

"If it's very short then that's okay, but if it goes on for a longer time we could have problems. At the end of January we had $6.5 million worth of roading damage and that's what we're very susceptible to," Mr White said.

"There's a chance that it may not be any more than a normal rain event, but having said that, I've said that before and then it turns nasty on you."

Northland Regional Council civil defence officer Graeme MacDonald said the rain was unlikely to cause any significant problems there, but the council had contacted owners of properties which tended to flood.

The weather was expected to clear today, but for intermittent showers.

THE OUTLOOK

* 100mm-150mm of rain for Bay of Plenty in 15 hours.

* 120mm-180mm expected for Coromandel ranges.

* High tide expected in Auckland at 10am and 11pm.

* Heavy rain warning for Auckland issued.