If you were rudely awakened this morning you weren't the only one.
Booming sounds woke residents from a serene Sunday sleep in. Several contacted the Herald inquiring about the mystery noise from as far afield as the East Auckland suburb of Stonefields.
One resident said it sounded like a massive air horn around 6am.
"One long beep then a break then another long beep just over and over and over."
Ships are believed to be the culprits by using loud horns in an attempt to navigate their way through the heavy fog.
The Auckland Harbour Master's office confirmed this.
"When there's fog, ships will be using their horns for navigation."
A Ports of Auckland harbour control spokeswoman said they had one inbound and one outbound ship this morning, and the ferries would be using fog horns as well. There is also a fog horn on the harbour bridge.
"It's just standard. It's lawful for them to use horns in fog. It's for other shipping so they know they're in the vicinity."
Fog is enveloping much of country and delaying international and domestic flights.
A heavy blanket of fog is sitting over Auckland, Waikato, Waitomo and Taumarunui.
MetService meteorologist Sarah Haddon expected it would lift around midday.
"It just takes awhile for the sun to come out and heat up the air to burn off the fog. As the wind picks up that will help."
Fog restrictions were in place at Auckland Airport but lifted at 8.30am.
Seven domestic regional flights have been cancelled and 10 were delayed. International flights VA110 from Gold Coast and VA90 from Rarotonga have been diverted to Wellington Airport.
Yesterday heavy fog caused 60 domestic regional flights in and out of Auckland Airport to be cancelled and 48 delayed.
Police are urging drivers to take extreme care as fog has again set in, in the same area where four people died in a crash in the Hauraki District last night.
Four people died after a horror car crash near Netherton just before 8pm as police warned motorists that fog was heavy in the area.
WeatherWatch reported that the fog was being caused by a strong high crossing the country.
"Northerly quarter winds develop across Sunday which should hopefully end the foggy conditions for most travellers."
It was a nippy night throughout the country with overnight lows plunging to -4°C at Lake Tekapo and Mt Cook.
Auckland reached 7°C, Wellington's low was 5°C and Christchurch got down to -3°C.