At The Warehouse, Frozen is the top costume choice.
"Our top-selling costume this year is the Frozen Deluxe Dress Up Costume, which has been flying off the shelves," a spokeswoman said.
Internet search analysis company SLI Systems found Frozen characters topped the list of most-searched-for costume ideas.
With more than 1.1 million searches for Anna, Elsa or Olaf, the choice eclipsed the next most popular options - zombies and ninjas, both at 863,000, and pirates, at 796,000.
Adults were sticking to zombies, vampires and anything "dead and scary", said the costume director at prop and costume hire shop First Scene, Jacqui Whall.
"People aren't getting dressed like the specific characters. There are all different types of vampires going on - people can choose from Underworld or Twilight or comedic vampires, like Taika Waititi's film What We Do In the Shadows.
"We're selling freaky contact lenses, all different types of cosmetic scars and wounds so people can wear anything and then make them dead and scary."
Costume Magic's Kathryn Hart said the Auckland business' phone had been "ringing off the hook" as adults and children made their costume arrangements.
Little Red Riding Hood had been a popular choice for girls, and boys were seeking Wolverine and Captain America outfits.
The Warehouse spokeswoman said Halloween's increasing popularity meant "double-digit growth" in sales of decorations.
Individually wrapped sweet treats were also big sellers as people stocked up for trick or treaters, she said.
Mrs Whall said customers were increasingly hiring props to decorate their homes and businesses for optimum scare factor.
"Pretty much all of our Halloween props were booked out weeks ago with people hiring old coffins, mummies, spider webs and Egyptian scarcophagi.
"New Zealanders are used to being straight up and down but now we are getting dressed up more and more for things like the Sevens and now the Nines, so it's opening the door for a whole lot of people."
Nifty trick takes guesswork out of locating treats
Orange balloons are being attached to letterboxes in one North Shore neighbourhood so trick or treaters know where they are welcome.
The Hensel family in Narrowneck put notes with balloons in their neighbour's mailboxes, asking them to hang the items if they are happy to participate in Halloween festivities. Accompanied by their parents, brothers Kyle, 12, and Chrishan, 8, will each take a friend with them around those houses tonight.
"We have gone around knocking on people's doors in the past and lots of people don't like it in New Zealand and the kids get a bit disappointed going from house to house and being told 'no'," said their mother, who did not wish to be named.
"We used to live in Newmarket and a friend of mine did it there and it worked really well, so we thought we'd give a whirl this year."
The notes - decorated with jack-o-lanterns, ghosts and lollies - went into 75 letterboxes along two roads, and invited neighbours to participate, or discard or return the balloons if they were not interested.
6 Tips for trick or treating
1. Decorations
Go only to houses that are well lit or decorated with Halloween accessories.
2. Knock only once
Politely knock or ring a door bell just once.
3. Say thank you
Say thanks for any treats, and be prepared to sing, dance or tell a joke if asked for a "trick".
4. One is enough
Don't: Ask for more, to swap or comment on the poor quality of a treat.
5. 14-year age limit
Trick or treating is really only for children; anyone over 14 should reconsider (unless they are supervising youngsters).
6. Stay together
To be safe: Check all treats, stay in well-lit areas, don't enter strangers' homes, and stay with the adults and friends you set out with.
