Citing personal communication from a paper
The particular paper (Warton et al. 2012) the OP is talking about is 6 years old. After such time, the research it referred to may not necessarily be still ongoing - it is likely to be either completed, or abandoned altogether. It would be best to personally communicate with the person in charge of the mentioned research (i.e., M. J. Anderson in this case) in this matter. Either there will be a publication to refer to and cite (citing also the 2012 paper if it's still relevant), or one can just mention the personal communication like it was done by Warton et al.