Australia are playing their first test series in South Africa since the 2018 "Sandpapergate" scandal, when the tourists used sandpaper to illegally alter the condition of the ball in a bid to gain more swing, resulting in bans for captain Steve Smith, David Warner and Cameron Bancroft.
The scandal followed a series of ugly confrontations, including off-field fighting and on-field jostling between the two teams in a sport that prides itself on its etiquette.
Harmer believes the growth of Twenty20 franchise cricket, where international rivals regularly become teammates, has taken some of the sting out of such contests.
"I expect Australia to come out firing and being competitive, but I think the nature of T20 cricket is such that you're playing with a lot of players, a lot of the time," he told a press conference on Wednesday.
“Gone are the days where you had these huge rivalries, where you would play against one another every 18 to 24 months, and it created that sort of argy bargy on the field.
“I think T20 has softened that a little bit but I don't expect Australia to be soft by any stretch of the imagination.”
The 37-year-old Harmer, expected to be one of two spinners South Africa will use in the first test, said Australia were always extremely competitive and the series in South Africa would be closely fought.
“They fight to the death; their lower order always seems to score a lot of runs with a never-say-die attitude. I think it is quite a similar trait to South Africans. We're resilient, and we probably do our best when our backs are up against the wall.
“I think that was there for the world to see in the World Test Championship final,” Harmer said of last year’s clash at Lord’s where South Africa beat Australia by five wickets.
“I think it was almost a foregone conclusion that Australia were going to win that game, but our team found a way to get through it and to get the win.”