You're referring perhaps to NICK TATE who portrayed Alan Carter in Space 1999.
Nick portrayed the following characters
TNG: "Final Mission", as Dirgo
DS9: "Honor Among Thieves", as Liam Bilby
can you be more specific? which actor?
Yes, that's him. The Australian guy. There was also a reference to the Australians in the Star Trek: Deep Space Nine episode "Little Green Men".