In this work we study the decays $\tau^{-}\rightarrow(K\pi)^{-}\nu_{\tau}$ using an effective field theory constructed with dimension six operators with the SM degrees of freedom. The explicit framework is the SMEFT at low energies. Following this framework we have obtained three main results:\\

(i) we have confirmed that it is impossible to understand the BaBar CP anomaly associated with the channel $\tau\rightarrow K_{S}\pi\nu_{\tau}$. We have found an upper bound for the NP contribution slightly larger than in Phys. Rev. Lett. 120(2018) no.14, 141803, but still irrelevant compared to the experimental uncertainty by four orders of magnitude approximately;\\

(ii) we have shown that the bump present in the spectrum measured by the Belle experiment for the $K_{S}\pi^{-}$ invariant mass distribution near the threshold cannot be explained by heavy NP;\\

(iii) we constrain the NP scalar and tensor effective couplings using the decays $\tau^{-}\rightarrow(K\pi)^{-}\nu_{\tau}$ and we find that they are competitive with other traditional low energy probes like hyperon decays for the scalar and tensor cases and kaon decays for tensorial interactions (we cannot compete for the case of non-standard scalar interactions in Kaon (semi)leptonic decays).\\

Besides these three main results, we have also studied the effect of NP in several interesting observables like Dalitz plots, decay spectrum and forward-backward asymmetry. All these observables were calculated in the SM case as well, in order to be able to compare the way in which NP could manifest.