This is a Japanese live action show known as Ambassador Magma in Japan, first airing there in 1966 but was not until 1978 that it was shown in English around the world including Australia. It was originally dubbed into English under the title Space Avenger in 1972 keeping the original Japanese theme song but was not sold to any TV stations until 6 years later under the new name of Space Giants, with a new opening theme song. Though I recently received an email from Adrienne Lamm (a_lamm@msn.com) from New York who remembers seeing Space Giants via Channel 11 in New York during 1969-1971... This was backed up from Robin Mach (robmach@iglou.com) who remembers watching Space Giants and Johnny Sokko in New York during this time.  
'Martin Urquiza' (Martin Urquiza <brujomartin@prodigy.net.mx>)  from Mazatlan, Mexico remembers watching "Space Giants" (known as "Monstruos del Espacio"). in 1972 Saturday mornings on channel  "8" - a national broadcast station that doesn't exists anymore. 
Peter Fernandez was responsible the dubbing for the English version, after his success with Speed Racer and Marine Boy. The narrator was the same person who did Prince Planet but I have no information on whom it was.
The first episode opens with an attack on Earth by the evil Rodak who has already conquered most of the Planets in the Universe and plans to conquer Earth as well.
Methusan, a wizard of many powers with long white hair and beard brings to Earth two robots Goldar and Silvar from the 2rf Galaxy, dedicated to stopping Rodak and defending the world.
Goldar is a 50ft golden robot, 14 feet wide and weighs 20 tonnes and his wife Silvar, a silver coloured 5ft robot. They live with their son GAM (created by Methusan) in a volcano in Mt Olympian.
They all have large antennae that fire Gamma rays (lazer beams) and can all transform into rockets. Through an opening in Goldars chest he can fire Torpedoes.
Rodak sends out a whole range of monsters to conquer Earth but ultimately are all defeated by Goldar and his robot family. The original Japanese names are kept in the series. The main human characters are centered around a Japanese chain smoking reporter - Ita Mura, who works for a world wide newspaper and his wife Tomoko and son Mikko whom when in trouble blows his whistle given to him by Goldar to summon Goldar or members of his family for help.