The England captain's form comes off the back of his first ever 30-goal Premier League season, and Walker was effusive in his praise for the prolific striker.
"Harry is just a goal machine. I remember watching him at Tottenham coming up, a few times as a 15-year-old kid and he was just banging in goals then - and he's just carried that on season after season," Walker told IRN.
"I remember people saying he's going to be a one-hit wonder and he's just completely proven them wrong and blown it out the water.
"It's full credit to him because the work he puts on and off the field is first class and he adapts himself and dedicates himself to the game, so his rewards are coming from it."
England will face Croatia on Wednesday in their first World Cup semi-final match since 1990.