Too little sunlight perhaps?

The garden gets about 6 hours of full sun and other dahlias
have normal healthy growth. I thought perhaps I added too much
fertilizer mixed into the planting holes of some tubers, but have not added any more since.
Now and I am hesitant to apply any more and welcome any suggestions.

Did you pinch thm when they were 3 to4 pairs of leaves high?

I would say FEED THOSE GUYS! GIve them a good dose of fish fertilizer. If they don't grow good stems and leaves how are they going to manufacture flowers? I made the mistake of under-fertilizing mine this year and I won't do that again! I like
a good time release fertilizer when I plant them and the results of a soil test told me I did not need all the parts of that so tried just planting with blood meal for nitrogen and lime to neutralize our acid soil. My plants were half the size of normal and leaves developed spotty yellow areas I thought were virus. In August I gave them a good dose of Fish fertilizer and it took a month to see much new growth and the leaves turn back to a healthy green. By then it was too late to put on any more height. My flowers were smaller then normal also.

My plan for next year is to go back to my osmacote/compost/alfalfa meal at planting time, with a bit of blood meal to work before the soil warms enough to make the osmacote available. And if I see slow growth and yellow areas on the leaves mid summer they get another good dose of blood meal at that time. All extra additions of nitrogen will stop by the first of September so the tubers can harden off so they will store well over the winter.

That is for my garden on the American Northwest coastal area. Where are you at that you are wondering about this in December when our dahlias are asleep for the winter?

It is early in the Southern Hemisphere season yet, and a metre high is good. Do you know what varieties you are growing? Some get taller than others.

Hi LizaLily, yes it is a bit early to be concerned with Dalias at this time of the year on the U.S. Pacific north coast. However I have transplanted to the mid north east coast of Australia. It's pretty warm here right now and the region is engulfed in huge bush fires, some on a 150 Km front.

However, my Dalias are now fully grown, about four feet high, have a few small flowers, and lots of foliage. I added blood and bone plus a small amount of Rooster Booster weathered chicken fertilizer in the planting holes
which may be the problem. Next year I'll leave the roosters out of it.
Cheers and Thanks for your comments.