An inkjet printhead assembly includes an elongate support having a plurality of internal webs protruding from a base section to define a plurality of parallel ink supply channels; a shim mounted on the support and defining a plurality of rows of openings through which ink from respective supply channels is provided; and a plurality of elongate printhead modules mounted serially on the shim. Each module includes a carrier carrying a printhead. Each carrier defines a plurality of ink supply passages through which ink passes to the printhead from respective rows of the openings. Either end of each carrier defines complementary formations such that adjacent pairs of the carriers nest together. The plurality of internal webs protrude from the base section to define a semicircular recess in which the shim is received. The shim is received in the semicircular recess such that the each of the plurality of rows respectively align with one of the plurality of parallel ink channels.