Traffic on Auckland's northwestern motorway may have to contend with more than other vehicles in the next few days, with the risk of flooding from exceptionally high tides in Auckland's Waitemata Harbour.

Parts of the northwestern motorway were very close to sea level and the New Zealand Transport Agency (NZTA) warned motorists today high tides of up to 3.5m, nearly half a metre higher than normal, would begin today and last until Saturday.

The high tides happened in Auckland every six to seven years.

The motorway causeway and cycleway next to it between the Patiki Rd Interchange and the Rosebank Rd bridges were most likely to be affected.

NZTA said cyclists should avoid using the cycleway 90 minutes either side of the high tide.

NZTA state highways manager Tommy Parker said the motorway could be affected if the high tides were accompanied by strong winds.

There was a traffic management plan to direct traffic away from the causeway if it flooded.

He said the NZTA was planning to raise the causeway by 1.5 metres and increase the width of the motorway to provide extra lanes as part of the western ring route development.