Hello! You asked about the major themes in At Hiruharama and how they are conveyed.
1) The difficult life of pioneers attest to their tenacity, resourcefulness, and courage.
Fitzgerald conveys how difficult life is for pioneers by describing the forbidding landscape.
But whoever lived there had given up, because of the loneliness and because it was such poor country.
Despite this intimidating landscape, Tanner proudly tells his sister what he and Kitty have been able to accomplish when he writes to her about Kitty's impending labor.
We have now a hundred full-grown hens and a further hundred at point of lay, and a good stand of potatoes.
Transportation and communication options are extremely limited. Fitzgerald conveys this by the fact that Tanner has to ask Parrish for the loan of two racing pigeons. The Blue Chequers are known for being able to fly two hundred and fifty miles; Tanner just needs them to be able to fly fifteen miles to alert the doctor when his wife goes into labor. Tanner's resourcefulness and quiet courage are assets in surviving Hiruharama's daunting landscape.
2) A good marriage is a strength in difficult circumstances.
It is clear that both Tanner and Kitty have a good relationship. When he comes back from his errands in Awanui, he tells her that he should have taken her with him. She chides him that he only thinks so because he has forgotten something at the stores. Both Tanner and Kitty are solicitous of each other's welfare. He worries about leaving her alone while he is in Awanui, and Kitty stays calm during her labor so as not to worry her husband.
3) Never underestimate the smallest details of life or humble beginnings.
Fitzgerald tells us that Tanner took the afterbirth out with the trash. His one great oversight was that he had not recognized a second twin daughter; he had mistaken the little girl for the afterbirth. It was this little girl who grew up to become a lawyer "with a firm in Wellington." Fitzgerald conveys this theme best with the Tanners' motto "Throw Nothing Away."
4) Always be prepared.
When Tanner finds out that Kitty is pregnant, he makes a beeline for the doctor. Among other things, he wants to know all about the statistics of birthing mothers dying during childbirth and whether there are any other women nearby who could possibly aid in the birth of their child. Next, he negotiates a loan of two racing pigeons from a man named Parrish. He wants to make sure that the doctor can be contacted when his wife is in labor. Tanner does not let the lack of modern conveniences detract from his preparation for the birth of their child.
Hope this helps. Thanks for the question.