HTC's signalled that it'll launch a 64-bit ARM-based handset on September 4th. The surprising news is that this isn't a high range handset.
According to an HTC post on Weibo, HTC's going to announce a 64-bit based handset on the 4th of September. HTC hasn't stated which 64-bit chip it'll be running on, although it's likely to be the Qualcomm Snapdragon 615 based on Qualcomm's own statements that suggest that HTC will be the first to use its mid-range 64-bit chip. G For Games speculates that it'll be called the HTC Desire 820.
Based on the recent history of Desire handsets in Australia, it's likely that if we do see the Desire 820 locally, it'll be a locked budget handset. If you're after a premium 64-bit Android handset, it looks like you may have to wait a while longer.