There is provided a height adjustment mechanism for a manhole assembly, including a lower body with a plurality of axially spaced-apart, height-fixing support assemblies. The mechanism includes an upper body having an annular wall and an end flange member extending radially outwards from the wall adjacent a lower end thereof. The upper body is insertable in part within the lower body. The upper body is axially rotatable from the insertion position to a fixed position in which the end flange member is abutable with one or more of said height-fixing support assemblies. The mechanism includes a locking pin that couples to the upper body and inhibits rotation of the upper body relative to the lower body upon the upper body being rotated into the fixed position. The locking pin being shaped to extend at least in part into a groove of the end flange member of the upper body.