Governments, economists, and policymakers in India have long been concerned about India's unsustainability of public debt. Therefore, the present study seeks fresh evidence of India's fiscal sustainability from 1980-81 to 2020-21. This study employs linear (model-based) and three non-linear models (quadratic, cubic, and kinked) to examine the fiscal sustainability in India. The regime changes are quantified using a 'regime-switching model-based fiscal stability test' that does not consider the linear fiscal response function method. The findings from the analysis point to a statistically significant fiscal sustainability outcome for the linear model but not for the non-linear model. There is also the sign of a 'regime-switching' fiscal rule in India from 1980-81 to 2020-21. The unsustainable fiscal regime has been identified only in three periods: 1983-84 to 1993-94, 2009-10 to 2012-13, and 2018-19 to 2020-21. For other periods, sustainable regimes are evident. The study finds that India's fiscal policy satisfies the 'non-Ponzi game condition', as revealed by the regime-specific reaction coefficients and the average periods of fiscal regimes. However, the country's long-term fiscal sustainability has been jeopardised, as the debt-stabilising condition is strictly violated. Regime-based insensitivity, which results in an unsustainable fiscal regime lasting for more than 3 years, might expose the country to long-term fiscal unsustainability.