His baptism abstract:

First name(s)           Frederic William
Last name               Myers
Birth year              1833
Birth date              ? ? 1833
Baptism year            1833
Baptism date            23 May 1833
Place                   Prendergast
County                  Pembrokeshire
Country                 Wales
Father's first name(s)  John Powell Matthew
Mother's first name(s)  Mary Anna
Page                    86
Record set              Pembrokeshire Baptisms

The image of the register page adds:
Abode:       Cotts
Profession:  Esquire
_______________

In 1851 he was visiting a couple, Randle W and Sophia H Falconer at ww Bennett Street, Bath, Somerset.  He was aged 17 and a pupil at school.

He is not to be found in any later censuses nor can I find any marriage to a Julia.

But his death is on the New Zealand records:

First name(s) Frederick William
Last name Myers
Birth date 1833
Age at death 36
Registration year 1869
Registration number 1869/3879
Certificate ordering
To order, visit https://www.bdmonline.dia.govt.nz/NonHistoricRecords
Country New Zealand
Record set New Zealand Death Index

The NZ site has:

Registration Number  Family Name Given Name(s)       Date of Birth / Age at Death
1869/3879            Myers        Frederick William      36Y
_________________

His marriage is not on NZ records from 1855 to 1868.
_________________

The birth of their son, another Frederick William, might be this on NZ records:

Registration
   Number   Family Name  Given Name(s)      Mother's Given Name(s)  Father's Given Name(s)  Still Birth
1864/25095     McKenzie     Frederick William  Julia                   Alexander               -

And other sons could be:

Registration
Number        Family Name  Given Name(s)  Mother's Given Name(s)  Father's Given Name(s)  Still Birth
1867/13755    Myers        Arthur Henry        Julia Anna         Frederick William       -
1869/14445    Myers        John William Rout   Julia Annie        William Richard         -

Daryl Coup reports a third son, John, born in 1865 but died in Jsn 1866. No evidence found for his birth.  But this death is to be found:

Registration Number  Family Name  Given Name(s)  Date of Birth/  Age at Death
1866/3170            Myers        John                           2M
_________________

The marriage of the second Frederick William can be found on FindMyPast from NZ records:

First name(s)           Frederick William
Last name               Myers
Registration year       1890
Spouse's first name(s)  Sarah
Spouse's last name      Colton
Registration number     1890/3215
Certificate ordering    To order, visit https://www.bdmonline.dia.govt.nz/NonHistoricRecords
Country                 New Zealand

And the NZ index to this is:
Registration
   Number     Bride's Given Name(s)  Bride's Family Name  Groom's Given Name(s)  Groom's Family Name
1890/3215     Sarah                  Colton               Frederick William      Myers
——————

Frederick William Myers 2 died in:

Registration Number  Family Name  Given Name(s)       Date of Birth/Age at Death
1908/3003            Myers        Frederick William   53Y
__________

And the latter’s daughter’s birth can be found:

Registration Number  Family Name  Given Name(s)  Mother's Given Name(s)  Father's Given Name  Still Birth
1891/3893            Myers        Annie Jane     Sarah                   Frederick William    -  -
__________

The death of Julia McKenzie is not found:
from 1900 to 1915 with the surname of McKenzie
from 1900 to 1915 with the surname of Myers

Julia might have (re)married and thereby changed her name?  So how about this marriage record?

Registration
   Number     Bride's Given Name(s)  Bride's Family Name  Groom's Given Name(s)  Groom's Family Name
1908/4024     Julia Iso              McKenzie             Lester Raymond         Smith

But there is no death of a Julia Smith around 1912.
________________

Sarah’s death is:

Registration Number  Family Name  Given Name(s)  Date of Birth/Age at Death
1923/4572            Myers        Sarah          69Y
—————-

And I can look for Annie’s  marriage in 191i and giving:

Registration
Number        Bride's Given Name(s) Bride's Family Name  Groom's Given Name(s)  Groom's Family Name
1919/8832     Annie Jane              Myers                Frederick Charles      Mitchell
—————-

And the death of Frederick Charles is in:

Registration Number  Family Name  Given Name(s)      Date of Birth/  Age at Death
1968/47015           Mitchell     Frederick Charles  74Y
_________________

Their domestic partnership is confirmed by her name on the birth records of their three children.