It might be that biting, glacial chill that hits you when you're out fetching the Bay News in your jockeys.
It could be the almost overnight emergence of pea coats, scarves and umbrellas unfurling in Willow St.
Or it might be the marked absence of those little things that have become so familiar to us over these fleeting few months, like the red and yellow DHL shorts in Maunganui Rd and the girls who cycle around the Main Beach boardwalk in bright sun dresses with no helmets on, who maybe think that they're in a summer fashion shoot but are really just asking to be classified status zero (that's dead) by St John paramedics.
However it arrives, it's always a remarkable moment when the changing of the season is marked by one funny little subtlety - and it's never a cliche like a dead oak leaf falling on your shoulder, the first frost or a newborn lamb bleating in a daisy-white meadow.
For me it happened when I was lying awake in bed the other night.
It was quiet, too quiet. Then I realised what was missing.
It was that despicable, sinister hum that we think is the sound of tiny wings beating but really just the mosquito's own dastardly way of telling us: "Yeeeah, just go to sleep you big hamburger, that's it, go to sleep ... and make sure you leave that big, fat juicy toe poking out for me too, yeah, that's it ..."
I hadn't noticed how it had suddenly crept up on me, the death of what's arguably the best part of living in the Bay, and felt guilty for watching DVDs inside when I could have been snorkelling around Mayor Island, sipping margaritas in the hammock or jumping off Te Awanui Drive bridge.
So I made myself a list of five things I need to do before the pea coats and umbrellas unfurl and Tauranga goes into hibernation for another year.
1) Rip into a feed of Fresh Fish Market fish n' chips at Main Beach at sunset. The good people at Dive Crescent might make the best damn fish batter you've ever tasted - but until you've enjoyed it with warm white sand underfoot and Mauao looming overhead you'll never truly appreciate it.
2) Swim at Papamoa Beach. I almost got snagged by a surf caster at Maketu, I shamefully lost a body-surfing contest with two 9-year-olds at Main Beach and I'm pretty sure I swallowed a mouth full of sea lettuce at Cutters Cove, but I never did get around to charging Hasselhoff-style into the Papamoa breakers. I figure I've still got a few weeks that I try this without St John paramedics having to classify me status zero.
3) Barbecue a meat pack at McLaren Falls Park. This I did try to do but I couldn't find a cooker that worked, so what was supposed to be a nice twilight barbecue became potato chips and red wine. Consider that a complaint, Tauranga City Council.
4) Spend a whimsical evening out on The Strand. I'm talking about heading out for a few after-work beers at Syndicate, that turns into dinner at the Naked Grape, that turns into a few more beers at De Bier Haus, that turns into bouncing around to a cover band playing Counting the Beat at the Crown and Badger, that turns into trying to dance with disapproving strangers at the Buddha Lounge, that turns into a thumping headache before deadline the next morning. Unlike most of my workmates, this is a Tauranga summer institution that I haven't indulged enough in.
5) Play twilight golf at Omanu. A mate asked me if I was up for it a few nights ago. It sounds like a must-do, despite the likelihood of one of my golf balls finding its way through the windshield of a $100,000 Audi zooming along Maunganui Rd.
So that's all the things on my to do list. I plan to spend each sunny hour crossing them off. For the record, I also intend to continue fetching the Bay News in my jockeys - but somehow I think the neighbours will put a stop to that one before the cold does.