The Bridgewater Quay apartment development, originally scheduled to be finished at the end of August, will now be completed in the next six to eight weeks.
The development on Taupō Quay has 20 apartments, all of which have been sold.
Developer Jon Hay said supply chains had been severely disrupted by the Covid-19 pandemic, both in New Zealand and around the world.
The country's recent Delta outbreak and ensuing lockdown had slowed proceedings as well.
"It's a major issue," Hay said.
"We've gone through stages where we can't get basic timber. Whanganui is surrounded by 10,000 hectares of forests and you can't get a stick of four-by-two in town.
"The boys have had to drive to Hāwera or over to Marton to get it, and that's a hassle."
Most of the workers on the Bridgewater site were sub-contractors, and had already committed to other upcoming projects, Hay said.
"If everything stops, suddenly, instead of having one job to complete, they've got three or four to do at one time. They'd do a bit on our job, and a bit on somebody else's and so on.
"It's quite disruptive for everybody, and the end result is that we're running late.
"There's nothing we can do about it. It's worldwide, that's one thing we know. Whether it's in the States or in Australia, everybody tells me it's the exact same story."
The price of steel had increased by 70 per cent in the past year, and timber had gone up by 25 to 35 per cent, Hay said.
"Those are massive increases for 12 months. You can pull your hair out and jump up and down, but that doesn't help. You've just got to persevere.
"I think 90 per cent of all New Zealand's logs are being exported to China, which is fantastic on one hand, but then we might be left with the knotty old ones at the sawmill.
"The building industry has enough challenges, so it's probably a good time for the powers that be to look at all this and make sure we don't get caught out like this again."
Hay said the development would have been held up even more if they hadn't learned from last year's initial Covid-19 lockdowns and pre-purchased a lot of equipment.
"If we wanted to go out now and buy 20 appliance sets for the kitchens - the ovens, hotplates, dishwashers, all the air conditioning units etc - you wouldn't have a snowball's chance in hell.
"We were lucky that we saw that one coming, but we didn't anticipate day-to-day building materials being in short supply. That's hit us a bit."
An issue that had saddened him was that a number of older people had already sold their houses in anticipation of moving into Bridgewater Quay, Hay said.
"It's troubling, and it's not something that's pleasurable. We'd like to get them in and get them settled, so we're really pushing like hell to make sure that can happen well before Christmas.
"Even though it's delayed, everything is really looking great, and it'll be something we can all be happy with and proud of when it's finished.
"It's being built properly."
Hay said he hoped that when Bridgewater Quay was completed, it would encourage other people to "get on and do things" with other sites in the city.
"We're seeing quite a few old buildings that were vacant and derelict now being upgraded and renovated.
"Hadleigh [Reid] has done a fantastic job, and then you've got the building across the road from us [6-8 St Hill St] that's being turned into apartments.
"There are some fantastic opportunities in Whanganui. The place has got a really bright future."