Anyone can witness your will, except for 2 important categories of person-the trustees and executors, and the beneficiaries. this extends to relations and partners- The Wills Act says -

Witnesses affected by dispositions made to them
(1) A disposition of property in a will is void if—
(a) the disposition is to a witness; or
(b) the disposition is to a witness's wife, husband, civil union partner, or de facto partner; or
(c) the property would go to a person claiming under a person described in paragraph (a) or (b).
(2) Subsection (1) does not apply if—
(a) the will has at least 2 witnesses who are not described in the subsection; or
(b) the disposition is the repayment of a debt to a person described in the subsection; or
(c) all the persons who would benefit directly from the avoidance of the disposition—
(i) consent in writing or electronically to the distribution of the property; and
(ii) have legal capacity to give consent; or
(d) the High Court is satisfied that the will-maker—
(i) knew and approved of the disposition; and
(ii) made the disposition voluntarily.

It is not fatal if this is not kept to as you can apply to the High Court to make the will valid, but it is best to get it right when being witnessed rather than gp through the costs of a High Court application and the risk that the judge may not give the order.