Australia has agreed to be an "accredited representative" of Malaysia
That's not how I read it.
Australia is not representing Malaysia, poor choice of words on the journalist's part. Australia is an accredited representative to the investigation, which is only to be expected per ICAO Annex 13, 5.23:
"Any State that provides an operational base for field investigations, or is involved in search and rescue or wreckage recovery operations ... may also be invited to appoint an accredited representative to participate in the investigation".