I think you can discount the "Henty" Sidney Haycroft :
The 1931 Electoral Roll , State division of Henty , actual address is 23 Burgess Street , Bentleigh has 6 Haycrofts there
AnnieHouse Duties HarryBuilder Ronald HenryDriver Stanley Sandry Musician SydneyDriver Sylvia BessieHouse Duties
From Trove , National Library of Australia ( Same as NZ Papers Past) :
Harry died 18/1/1933 , husband of Annie , brother of Bessie, Gracie, Jack, Ethel, Reg & Roy
Also Son of the late John (Died 1901 aged 50) and Mary Jane (died 1941) Haycroft. (both of these buried Nhill Vic, Cemetery)
Annie Haycroft died September 1947 , mother of Stanley, Ronald, Sidney & Vernon.
From Ryerson Index Stanley Sandry Haycroft died 2005 aged 101.
From Victorian BDM's ( Sadly Victoria likes to charge you for searching the index , not like NSW which is free) I found a birth of a Sidney Haycroft between 1900 and 1910 , and was able to reduce in bit by bit to 1907. As you cant view it you must get a " Match" , so by using Harry and Annie as parents for 1907 Sidney , you get a match. There is also a marriage for Sidney in 1932 , but no death to 1988 , and nothing on Ryerson. Since his bother lived to 101 , he could still be alive at aged 107. Tried both Sidney & Sydney , and Haycroft, Haycraft, Hoycroft & Hoycraft.
So pretty sure this is not yours . there are very few Haycrofts in Australia , slightly more Haycrafts.
I note there are no births in NZ for Sidney & Nga Waka , could they have moved to the Cook Islands or any of the other Islands for peace as he seemed to have had problems being invovled with a Maori >
There is no trace of him in NSW , in fact only a couple of Haycrofts totally