Motorway flooding risk with king tides

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Sections of the Northwestern cycleway will most likely be underwater and low-lying sections of Auckland's motorway network are at risk of flooding during high king tides forecast for later this week.

Exceptionally high king tides forecast for the Manukau and Waitemata harbours risk flooding lower sections of the city's motorway network, the NZ Transport Agency warns.

In the Waitemata Harbour, high tides of up to 3.7 metres - about 0.6m higher than normal - are expected from this Friday until the following Tuesday.

Usually, king tides occur over a period of two days and peak at 3.6m.

Areas most at risk from flooding include a section of highway and adjoining cycleway on the Northwestern Motorway (State Highway 16) causeway between Great North and Rosebank roads, and the Northern Motorway (SH1) near the Onewa Road interchange.

In the city, some parts of Tamaki Drive could also flood, the transport agency warns.

The agency's national journey manager, Kathryn Musgrave said the king tides would be at their highest on Saturday and Sunday morning.

"Our advice to all road users, including cyclists and walkers, is to take extra care - especially during that hour-and-a-half either side of high tide.

"If necessary, we will divert drivers on to other roads to allow them to continue their journeys safely, and sections of the Northwestern cycleway will most likely be underwater which will make walking and cycling hazardous."

The Transport Agency's SH16 Causeway Upgrade Project includes raising the section of the Northwestern Motorway 1.5 metres to prevent future flooding, Ms Musgrave said.

The king, or perigean, tides occur when the moon is either new or full and closest to the earth. Low pressure weather patterns and strong winds can also influence the height of the tide.