Rare flavour-changing neutral current processes are sensitive probes for physics beyond the Standard Model. The ATLAS experiment at the Large Hadron Collider (LHC) has performed a measurement of the branching fraction of $B^0_{s}\to\mu^+\mu^-$ decays and a search for $B^0\to\mu^+\mu^-$ decays with data collected in 2015 and 2016, in combination with a previous result by ATLAS on data collected in 2011 and 2012. The results are combined with recent results by the CMS and LHCb experiments. The angular decay distributions of $B^0\to K^{*0}\mu^+\mu^-$ decays are measured with 20.3 fb$^{-1}$ of data collected in 2012. For both analyses, the expected improvement in sensitivity and precision is estimated for the High-Luminosity LHC.