So perhaps the 1884 birth is to this couple.
Marriage
6975/1883
HOLEN Peter
HARMER Harriet R
District Shoalhaven
I wonder if the Holen-Harmer marriage is the same couple also, although I am not sure about how the dates match.
His parents as listed on his marriage certificate are William Peter Hollands (Detective)
and
Harriett Rose Anna Fuller, both deceased as of 1918.
Don't know if this will help you decide if the 1883 marriage is the right couple and
if this Harriet Rosina Harmer is Peter William Holland's mother
Harriet Rosina HARMER Apr-May-Jun 1863 Battle Sussex Vol 2b Page 50
a pedigree tree (familysearch) says
Ellen Augusta Harmer had 3 children
all christened Battle Sussex, Surname HARMER
Ann 10 Apr 1861 burial 21 Apr 1863, James Frederick Samuel 3 Nov 1861 &
Harriet Rosina 2 Aug 1863
https://familysearch.org/pal:/MM9.2.1/MZ4M-94J?view=basic1871 RG10; Piece: 1037; Folio: 18; Page: 3
No 3 Part cottage Brightling Sussex
John
FULLER 40 51 Stone drawer
Ellen 31 Ashburnham SussexAnn 28
Stephen 17
John 11
James HARMER step son 9 Ashburnham
Harriet HARMER Step dau. 7 Battle Sussex
Philadelphia FULLER 5
George Fuller 2
Robert Fuller 9 months
Jane Fuller gd/dau 4 months
1881 RG11; Piece: 1034; Folio: 29; Page: 12
John & Ellen FULLER family plus step son James HARMER
https://familysearch.org/pal:/MM9.1.1/XQ9H-MB4Harriett HARMER born c1861 Sussex occ Kitchen Maid
Arrival Date: 19 May 1883 Sydney NSW
Vessel Name: Lochee
Departure Plymouth
Possible second marriage for Harriet
Harriet R HOLLAND Samuel SOWDEN
Marriage Date: 1889 New South Wales
Reg Place: Maclean, New South Wales Reg number: 4193
This looks to be Harriet's mother
1861 RG 9; Piece: 565; Folio: 148; Page: 15 – Head Henry Champion 30
Mill corner Ashburnham Sussex
Ellen FULLER 22 Ashburnham Sussex Servant Hs Keeper
probable marriage entry
(1891c shows Ellen Fuller born Ashburnham with middle initial
'A')
Ellen A HARMER Mar qtr 1864 Battle vol 2b page 48(a)
John
FULLER Mar qtr. 1864 Battle Vol 2b page 78(a) - The expected page range for this district is 43 to 55
probable birth/death entries
Ellen Augusta Harmer Sep 1839 Battle Sussex Vol 7 Page 218
Ellen FULLER died 1904 age 64 Battle Sussex
FOR INFO.There are a number of public trees on ancestry for
Harriet Rosina HarmerBirth 21 Jun 1863 in
Battle Union WorkhouseDeath 28 Aug 1958
mother
Ellen Augusta Harmermarriage to
John Fuller 16 Feb 1866 St Thomas a Becket, Brightling East Sussex
there are pictures of some of the Fuller children
23 Apr 1899 Presbitarian Church, NSW, Australia no place given
Samuel SOWDEN of Lismore, farmer married
Harriett Roseyina HOLLAND age 35 of Casino, dau of James Harmer, Battle, Eng.
you would need to verify info.