City meet Spurs having lost their past four games, but will have received a psychological boost with Guardiola's commitment.
Postecoglou said, "It is tough yeah, you are playing City at their place and they have a pretty imposing record there. Those four losses have all been away from home. Irrespective, it is always a great test and a great challenge for us to go there and see whether we can disrupt things again and hopefully put some pressure on them but we know it is never an easy task.
"It is great that Pep has re-signed. It is fantastic for the Premier League. He keeps setting the standard which is great for all of us trying to mow him down."
On Guardiola's influence on the game, Postecoglou also said: "I think in many varied and different ways. One is through a sustained excellence. It is not easy to win in different countries and I know that he has been at big clubs but again, as we have seen recently, that is not necessarily transferable to winning things all the time. You still have to do it.
"So he has maintained a ridiculous level of excellence. The way his team plays, they are always challenging every opponent they face and just his energy to maintain that for so long is outstanding. He will go down as one of the greats already but the indication that he has resigned probably means that there is more to come."