According to the Museum Victoria Website over 40 different medals were made to commemorate Cook's Bicentenary in 1970. (sounds like something to start collecting )
They have examples from the Daily mail, House of Hawke, State Savings bank, Solvol Soap, PJ Downie. etc.... (but no National Trust ) most of them minted by Stokes.
Noble Auction of Australian Historical Medals
lists James Cook 1970, National Trust Medal, extrememly fine in case for $50
(no picture but I'm assuming it's the same)
Also it's 200th...not 300th