Where in On the Beach does Peter Holmes mainly appear?
This is difficult to answer in that page numbers will be different depending on what copy you have. Instead, I'll give you chapter numbers where Peter's character is shown through his actions and dialogue.
Peter is introduced in Chapter 1, and we're given a description of the kind of man he is as well as the background information for the book.
In Chapter 4, Peter and Mary plan their gardens, both flower and vegetable. This tells us a great deal about the couple since, as Moira points out, they won't be around to enjoy the fruits of their labors.
In Chapter 5, Peter expresses his fears to Mary and tells her all about the effects of radiation poisoning, saying they might have to committ suicide to save themselves from long suffering. They have a terrible fight, but in the end, they make up.
Chapter 9 is the heart-wrenching last chapter where Peter and Mary kill not only themselves, but their baby as well. Towers sails away on his death mission, and Moira watches his ship and kills herself as well.