Morelia harrisoni is separated from these three species by distribution, being the only 'Carpet Python' known to occur in New Guinea. The specimens allegedly from north of the Central Dividing Range in New Guinea are also believed to be of this species. In appearance, Morelia harrisoni usually looks somewhat intermediate between Morelia variegata and Morelia cheynei although Morelia harrisoni is highly variable in colouration, both between different localities, within the same locality and even within a single litter of offspring. In line with Morelia variegata, Morelia harrisoni young are often reddish in colour. Morelia harrisoni is one of the smaller varieties of Carpet snake.
Adults average slightly less than two metres in total length.
It is not known by this author to exceed three metres in length. The Morelia harrisoni sighted by this author tend to have a lower average ventral and subcaudal scale count than Morelia cheynei, Morelia variegata and Morelia macdowelli, however the sample seen is too small to conclude if this trend is general. They also provide photos of this species in life. However again this author notes the general variability in colour in this species and the fact that colouration does change with age. Like all Carpet Pythons Morelia harrisoni is almost always dorsally patterned with a combination of cross-bars, blotches and spots, occasionally in a manner approaching longitudinal blotches or stripes.