"The week before last I had nearly 25 available properties and everybody wants them rented before Christmas," he said.
"There's a lot of people in the same boat ... hence the reason I'm at work early every day.
"Just trying to, while I've got all these properties, just trying to help out everyone out who's been given notice and things like that."
He said Property Brokers had about 17 rentals available now.
"There was a massive squeeze but what's happened is a lot of investors have bought properties earlier in the year and they've been doing work on them so they've all come up now ... which is not ideal.
"That's why we've probably got more than most of the others. There's still a squeeze for a smaller size ... the smaller unit size properties."
Rennie recently helped mum of six, Melissa Walsh, get into a rental in Castlecliff after she was told she would have to move out of the Whanganui East house she had been in for the best part of a decade.
It was still tough, Rennie, said for people like Walsh who would face a big hike in the prices they were paying for rent.
"Of course the rents are ridiculous for a lot of the properties. They've gone up so much [for] the people that've been given notice that have been in properties for years ... they start looking and the rents are way more than what they had been paying. That's more the squeeze, it's that they can't afford some of the rents."
Elsewhere, Landlord's Link could not say the same of the rental property market in Whanganui.
Its managing director, Tracey Onishenko, said her organisation had seven available rentals. She said demand also seemed to have cooled at this time of year.
Onishenko suspected Property Brokers had a lot of investors' rental contracts because they were also involved in sales, whereas Landlords Link is solely a property management company.