The horses run free on New Zealand Army land near Waiouru and need to be kept at a population number of about 300; the minimum number for maintaining a genetically viable herd and protecting the surrounding environment.
Ms Frewin said so far 16 applications for 28 wild horses had been received and encouraged people both experienced and new to muster horses to apply to rehome them.
"If people have got the facilities, time and experience it's definitely something that is very rewarding. It's been really interesting to see the number of people who have taken horses before and are applying again this time around.
"To have people who are now becoming regulars and taking horses every muster says a lot about the horses themselves and how fun they are to work with. They've all got big personalities and every one is completely different to work with."
Horsewoman Tracey Thompson has taken on several Kaimanawa horses every muster since 2012, rehoming 20 horses in 2016.
"It's hugely rewarding, I love it. They're a blank canvas because they haven't been touched by people.
"You're starting from scratch and anything we teach them they take on board so they learn really quickly once they get over their fear."
The Takapau woman rehomed the somewhat famous Kaimanawa horse "Brat" who was rescued as a 2-month-old foal.
Now being ridden with a saddle, Brat was exceeding expectations and a real character around the farm, Ms Thompson said.
"Anybody who has got experience with horses would find it hugely rewarding. It's an amazing journey to go through from taking on a completely wild horse to the quiet ponies that they can become.
"I've got ponies now that I jump on with just a halter and ride around on the farm bareback. They're so cool but they're certainly wild when they arrive."
Although she wasn't taking on any wild horses permanently this year, Ms Thompson said she was able to handle up to 10 horses straight from the muster for people without the facilities or experience.
"It is going to be a big muster this year so we need all the people we can get to take them on."
This year's Kaimanawa Horse Muster will run from April 18-22 and all horses not rehomed will go to the slaughterhouse, Ms Frewin said.
"Everything that comes into the yards leaves on a truck, one way or another."
For more information about the 2018 Kaimanawa Horse Muster visit www.kaimanawaheritagehorses.org.