We study the full field and frequency filtered output photon statistics of a resonator in thermal equilibrium with a bath and containing an arbitrarily large quartic nonlinearity. According to the general theory of photodetection, we derive general input-output relations valid for the ultra-anharmonic regime, where the nonlinearity becomes comparable to the energy of the resonator, and show how the emission properties are modified as compared to the generally assumed simple anharmonic regime. We analyze the impact of the nonlinearity on the full statistics of the emission, g((2)), and its spectral properties. In particular, we derive a semianalytical expression for the frequency resolved two-photon correlations or two-photon spectrum of the system in terms of the master-equation coefficients and density matrix. This provides a very clear insight into the level structure and emission possibilities of the system.