Hi thenall,
The following should help in your efforts to find family who may like Frank's diaries:-
Frank Simon's marriage notice
https://paperspast.natlib.govt.nz/newspapers/THD19170510.2.19?query=frank%20simonA 1925 Electoral roll shows Mamie Simon as a widow of 56 Oriental Parade, Wellington.
Horace Simon's marriage notice
https://paperspast.natlib.govt.nz/newspapers/ODT19180816.2.15?query=horace%20simonThe birth of a son to Horace and Phyllis
https://familysearch.org/ark:/61903/1:1:2F14-HDDThere are immigration records between New Zealand and Australia for Frank Derek/F D Simon on FamilySearch eg
https://familysearch.org/ark:/61903/1:1:QJDK-X8BFThere is reference to his life story on the following link
https://www.flickr.com/photos/41311545@N05/5614396376Derek, as he was known, married Lorna Gibbs on 21 July 1949
http://trove.nla.gov.au/newspaper/article/138504420?searchTerm=%22mrs%20a%20paykel%22&searchLimits=sortby=dateDescThe birth of a daughter is announced in The Sydney Morning Herald in 1953.
Horace passed away in 1984 and Phyllis in 1989
https://billiongraves.com/grave/HORACE-SIMON/9329185#Brothers Frank B: 1890 and Horace B: 1894 also had a sister Fanny/Frances who was born to Edith nee Mat(t)hews and Thomas Simon in 1896.
Fanny's marriage notice - note she married 'Abraham' Paykel not Alf as in the notice
https://paperspast.natlib.govt.nz/newspapers/EP19270114.2.3?query=paykelThey made their home in Australia and the 1935 NSW electoral roll gives their address as 2A cooper St., Strathfield. The birth of a daughter is announced in The Sydney Morning Herald in 1931 and the engagement of their eldest daughter in 1947.
There are immigration records for Fanny/Frances and/or Abraham on FamilySearch eg
https://familysearch.org/ark:/61903/1:1:KL1R-MLJIf you click back one page on this one and look at the further details for Abraham, passenger 15, there is a new address of 21 Frances Street, Lindfield, Sydney.
https://familysearch.org/ark:/61903/1:1:KXH4-W3CAbraham, who worked in the family Paykel Bros Company, passed away in 1956
https://familysearch.org/ark:/61903/1:1:JVXW-8M9and Fanny in 1990.
https://www.heavenaddress.com/restingplace/frances-paykel/773860Mother Edith Simon passed away on 12 Jan 1948 and father Thomas on 12 May 1955
https://billiongraves.com/grave/person/9103998#Edith's will is on this link
https://familysearch.org/ark:/61903/1:1:QK9V-KK8Vand Thomas', which also mentions a named grandson, on this link
https://familysearch.org/ark:/61903/1:1:QK9V-28S5