The work investigates a magnetic fluid filled photonic crystal fibre (MFPCF) at 1557 nm under applied magnetic field. They computed the refractive index change of the MF due to the magnetic field and used it in simulations to find the effective refractive indices of the core and the first cladding modes of the MFPCF. Then, they analytically computed the effective mode area and the confinement loss. The effective refraction index of the core mode increasing by 0.0341% under the applied field of 0-500 Oe, increases the effective mode area by 128% and the confinement loss by 866%. Thus, a rather small change in the effective refractive index gives rise to a rather large change in confinement loss. However, this effect saturates after about 500 Oe because the MF nanoparticles no longer change their magnetic orientation. It is also noteworthy that the confinement loss changes almost linearly with the effective area as the magnetic field change. These results suggest that MFPCF are suitable for a tunable attenuator and a field sensor.