Auckland motorists are being reminded to check their traffic route before heading out on Sunday morning.
Nearly 14,500 runners are taking part in the Auckland Marathon, which starts in Devonport, heads across the Harbour Bridge and wraps up at Victoria Park.
Race director Bec Williams says if you're not taking part, planning your journey is essential.
"Leave a little bit earlier, and look at those diversions and ensure [you] can get wherever [you] need to be on time."
Changes to bus timetables - too many to list here - are available on Auckland Transport's website.
The first race began at 6am. The super city is expected to hit 14degC in the morning, with 90 percent humidity.
"We'd encourage people to make use of those aid stations, where there will be water, and the last few aid stations… will have Coke on board, so certainly stop, take a drink and make sure you stay safe and well hydrated," says Ms Williams.
But the official Auckland Marathon Twitter account called it "perfect running conditions" as races began.