This paper presents a new measure that can be used to identify the locations where high levels of HVDC modelling can be simplified while maintaining the accuracy of the small-signal stability results. Low-frequency inter-area oscillations have posed a major issue to power system operations and stability, for example by reducing the maximum power transfer on tie lines connected between different regions or causing cascading failure due to ever-growing oscillations. In modern power systems, a significant number of High Voltage Direct Current (HVDC) systems are connected to power networks. An appropriate level of HVDC modelling is required in order to obtain accurate small-signal stability results in multi-infeed HVDC systems. Although accurate results can be obtained by using complex modelling details, this can be prohibitively computationally expensive due to increasing simulation time. In this paper, different levels of modelling fidelity for HVDC systems are investigated in order to establish the impact of the included HVDC system dynamics on smallsignal stability. It is shown that some dynamic HVDC modelling, particularly LCC-HVDC and VSCHVDC with P-Q control, can be replaced by simplified models in order to reduce the simulation time and model complexity while maintaining the accuracy of small-signal stability results. Furthermore, a method to identify which HVDC systems require detailed modelling based on the quantification of changes to critical mode shapes is developed.