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As radio polarised emission from astrophysical objects traverse through foreground magnetised plasma, the physical conditions along the lines of sight are encrypted in the form of rotation measure (RM). We performed broadband spectro-polarimetric observations of high rotation measure ( | RM | ≳ 300 rad m − 2 ) sources away from the Galactic plane ( | b | > 10 ∘ ) selected from the NVSS RM catalogue. The main goals are to verify the NVSS RM values, which could be susceptible to n π -ambiguity, as well as to identify the origin of the extreme RM values. We show that 40 % of our sample suffer from n π -ambiguity in the NVSS RM catalogue. There are also hints of RM variabilities over ∼20 years epoch for most of our sources, as revealed by comparing the RM values of the two studies in the same frequency ranges after correcting for n π -ambiguity. At last, we demonstrate the possibility of applying QU-fitting to study the ambient media of AGNs.