During the 1978 New Zealand rugby union tour of Britain and Ireland, he completed a "Grand Slam" with the four Home Nations.

Johnstone continued to develop as a prop, earning a spot on the North Island team in 1975.

From 1977 until 1980 he was a regular member of the team, gaining respect as a "compelling scrummager" and for being "extremely mobile with great ball skills".

He was an outstanding member of the first All Blacks team to complete the Grand Slam, beating all the home nations in 1978.

Under his stewardship, the Fijian national team gave France an almighty scare in the quarterfinals before going down in the 1999 Rugby World Cup.