ITER First Plasma (FP) operation aims to produce a plasma with current higher than 100 kA for a duration longer than 100 ms. However, the low values of plasma density and temperature associated with the initiation phase make it difficult to diagnose the plasma accurately. It is therefore essential to develop models for the available diagnostics for ITER FP operation in order to determine the necessary measurement ranges for plasma initiation and use them as inputs for controller development and assessment within the plasma control system (PCS). The use of accurate diagnostic models also helps to optimally prepare for, and analyse, ITER FP operation. A model developed for the Hα main chamber visible spectroscopy for FP shows that it is possible to measure Hα emission soon after breakdown for plasma temperatures higher than 3 eV and average plasma densities higher than 1017 m−3. The diagnostic model for the interferometer shows that for the proposed ITER FP scenarios, accurate measurements are possible for average plasma densities higher than 5 × 1017 m−3. The results from modelling of the hard x-ray monitor demonstrate that it can provide signals useful for runaway electron detection and can measure current levels typically down to 3 kA. Thus, these measurements can thus be used effectively by the PCS for runaway electron (RE) detection during FP operation. Improved diagnosis of the plasma state deducing, for example, parameters such as the plasma size and position, is possible by combining the measurements from the different FP diagnostics (density interferometer polarimeter and Hα main chamber visible spectroscopy), which will be available for ITER FP operation.