Almost 50 domestic flights around the country have been cancelled or delayed due to heavy fog in Auckland.

Auckland Airport has been urging passengers to check their flights details as heavy fog hits the terminal this morning.

Thick fog is blanketing many parts of the city early today, making for difficult driving conditions also.

8.40am update

The airport reported approximately 21 domestic regional flights have been cancelled and 31 domestic regional flights delayed due to fog.

Domestic flights to Wellington, Christchurch, Dunedin and Queenstown have not been affected. International flights have not been affected either.

Road authorities are advising drivers to keep their lights on this morning, as heavy fog remains in patches across the Auckland motorway network.

"With reduced visibility, please remember to keep your lights on, allow extra time and mind your following distance."

A crash was earlier blocking two southbound lanes on Auckland's Southern Motorway, between Tecoma St and the Ellerslie Panmure Highway.

Another crash on State Highway 1, just south of Warkworth, about 6.40am is now blocking both directions.

Waka Kotahi NZ Transport Agency said at 7.15am that there are currently "long delays" in the area and motorists are advised to delay their journey or consider using SH16 as an alternative route.

An update just before 8am advised that a stop-go traffic management system is now in place on SH1 between Valerie Close and Satellite Station Rd.

Ferry services delayed

Commuter ferry services in and out of the city centre have also been affected by the fog.

Passengers have been told to expect delays to all Hobsonville, Pine Harbour, West Harbour ferry services; as well as to the Gulf Harbour, Birkenhead and Northcote Point ferry, Bayswater and Devonport services.

One commuter heading into town from Waiheke said: "The 7.30am ferry from Waiheke to Auckland has slowed to a crawl through the fog and making regular soundings on its fog horn."

Heavy fog is affecting ferry services in Auckland. Photo / Bernard Orsman

Auckland Airport issued a message on its social media channels at 6am, saying: "Auckland Airport is currently experiencing fog.

"Passengers should check with your airline, the (airport's) website or the Auckland Airport app for the latest flight arrival and departure information."

The airport's website appears to be clogging up also, with a "sorry, we seem to be having some technical difficulties" sign popping up at times.

An Air New Zealand flight from Auckland to Palmerston North, scheduled for 6.30am, was cancelled; while another flight to Palmerston North has been delayed until 7.15am.

Two flights scheduled to leave for Nelson at 6.50am were both cancelled, while a flight to Tauranga has been delayed until 7.30am.