I thought the same thing at first, but as far as I can tell Tennant Creek Gold was one of the acquisitions along the way to the company becoming TNG:
See
https://www.marketindex.com.au/asx/tng
and this from the TNG website:
https://www.tngltd.com.au/docs/pdf/10-05-04%20%20Acquisition%20of%20Tennant%20Creek%20Gold.pdf
The ASX code history is towards the bottom of this page:
https://www.asx.com.au/asx/statistics/announcements.do?by=asxCode&asxCode=HLM&timeframe=D&period=M6#codehistory
With a big caveat being that I'm new to this so I could be wrong...