Traffic is easing on Auckland's motorways after bad weather and crashes caused jams this morning.

The New Zealand Transport Authority posted on Twitter that it was taking 40 minutes to get into the CBD after a citybound crash on the Northwestern Motorway after Royal Rd partly blocked the right lane and another crash after the Lincoln Rd on-ramp caused severe back-ups. The crashes have now been cleared, although a backlog remains.

A crash on Manukau Rd, near Epsom Girls' Grammar, Newmarket, and a crash eastbound on Highbrook Dr, East Tamaki, were also creating queues.

The jams come amid heavy rain in the city, and around the country. Stormy weather is expected to continue in the upper North Island this week.

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A heavy rain warning has been issued for Rotorua and Bay of Plenty, and a severe weather watch north of Hawkes Bay.

The MetService said a deepening low was expected to move southeastwards across the North Island, bringing a burst of heavy rain to Northland, Auckland, Waikato, Coromandel Peninsula, Gisborne and Hawkes Bay today.

Stormy conditions hit Auckland yesterday and up to 90mm of rain is expected in Bay of Plenty and Rotorua by midnight tonight.

Slips and surface flooding are possible, the MetService warned, and hazardous driving conditions are expected.

The weather service is forecasting heavy rain of at least 15-25mm during "thundery downpours".

More than 70mm of rain in 12 hours is possible for Northland from early this morning until late afternoon, in Auckland from this morning until evening, in Waikato from late this morning until this evening, in Coromandel Peninsula from this morning until this evening and in Gisborne and Hawkes Bay from this afternoon until early tomorrow.

The MetService is advising people in these areas to keep up to date with forecasts.