Barnacles after 5 months since anti fouling was done

Most likely cause is not preparing the surface properly before applying the anti fouling paint.
To do a lasting job you need to remove the build up of old flaky antifoul till you have one nice even surface, then primer, then at least 2 coats of antifoul.
The first step is the most important part.

If the boat is not moving much, stuff will still grow on antifoul, but its easier to get it off.
Check to see if it is:
-- ablative. Soft or self ablating, comes off as you move through the water. Hence it needs to be redone every 12-18 months as its all gone; or
-- hard. Doesn't come off as you move through the water. But after a few seasons you end up getting multiple layers you'll eventually need to strip back. Still redo it every 12 to 18 months.

You can wet sand hard antifoul or use coarse brushes, but ablative you need a bit more finesse. Scrub ablative with a hard brush and you'll end up swimming in a big blue/(insert antifoul color here) cloud of it.
I had a diver do a hull clean once a month. If I left it any longer than that you could feel it was a little bit sluggish from the slime when motoring or in light airs. Some guys don't get a regular hull clean at all and rely on regular sailing to keep it clean.
Horses for courses mate.

I always wet sand my boat to a very high standard every time I antifoul, she has no buildup of old paint and after 22 years I can still get back down to primer at some patches. I use 2-3 coats of Wattyl Seapro 120 and I always get some barnacles. I probably get out to about 4 months before one or two make their mark. I clean these off and give the boat a little scrub with some 60 grit. The Wattyl is a hardish paint, whch is why I use it, and can take the sanding. I would love a paint that lasts longer and is hard enough to sit the boat on the sand. I usually have to antifoul every 18 months but the last 6 months the swims get more regular.

Because my boat is a cat, it is very easy to take her to shallow water and gently rub her bum. It keeps me up to date with how she is going and I check her hull for any problems at the same time.

What antifoul did you use? Had similar issue with my most recent application.

Same 60grit wet paper on a sanding float, after the gurnie, it's a good way to find any dodgey bits and create a good key

Has anyone here used an air-powered sander? Around Port Stephens there's nowhere you can dry-sand antifoul with an electric sander, I have a LOT of old hard racing antifoul to remove, and neither wet sanding by hand or a scraper do anything to it. The alternatives seem to be to pay thousands to get it professionally blasted, or get an air tool.

Local marina guy ( Pittwater } told me there's an invasive species of barnacle about and that's why there is such rapid re-growth.

Yep air sanders need a lot of air, even the little palm sander, though you would imagine most slipways would have a good compressor and the air tools are relatively cheap. It's a gruesome task no matter how you look at it. I'm assuming a glass hull so burning it off would have complications. I recently had the task of blasting a big aluminium cat with an industrial pressure washer with a ceramic tip it took old anti foul off, but maybe too savage, likewise the needle scaler, so perhaps the chemical paint stipper, but as I said gruesome!

I have applied antifouled to approximately 200 + Vessels including my own over the past 30 years. Not once I have wasted my time wet rubbing the old paint back and always laugh when I see yachties arrive on the slip rubbing down the hull with wet rub and changing there facial skin and arms colour to black or blue.

A good high pressure wash is all that is needed for prep . Some scarfing in with scraper and 80grit maybe required with a compatible high build primer and tie coats if the antifoul coatings are more than 10 years old and flaking ..

At the end of the day it's just a single pack paint very much like repainting your bathroom in your house, it's not a two pack where you need to remove the gloss .