An INZ spokeswoman said it was aware of Mulitalo's circumstances, including her health and character matters.
"They will be will be carefully considered in the assessment made against the relevant immigration instructions," she said.
But her immigration lawyer Soane Foliaki said the historical issues should have no bearing on the decision INZ makes on Lomu's immigration status.
"It is unlawful for a decision-maker to take into consideration matters that are not relevant in its decision-making process," Foliaki said.
"What normally happens is that once a (partnership work visa) application is filed, an interim visa is granted to allow the applicant to remain on a valid visa until the actual application is decided."
According to Tongan court documents, Mulitalo - who was identified under her married name Sauiluma Guttenbeil - ran over her husband Michael Guttenbeil with her four-wheel drive vehicle.
Guttenbeil died after suffering severe internal bleeding in his chest as a result of broken ribs and injury to the arteries of the heart.
She was originally charged with murder, but was acquitted and found guilty of manslaughter.
Mulitalo received a suspended sentence at the Nuku'alofa Supreme Court on December 12, 2014, where she was also required to attend a Salvation Army anger management programme.
The Herald understands the organisation was instrumental to helping her and the seven older children move to New Zealand in 2015.
Mulitalo has been in a relationship with Lomu since 2017, and they have a three-year-old son. She is deeply worried that if he is deported, the eight children could be left orphans.
Lomu has been the primary caregiver for Mulitalo and the children aged between 3 and 18.
She described Lomu as "the rock" in the family and her pillar of support since she was diagnosed with colon cancer in September 2017.
The cancer had spread to her liver and Mulitalo fears she does not have long to live.
Lomu's first work visa application was declined and an appeal to the Immigration and Protection Tribunal was lodged in February last year.
The IPT found Lomu's case had exceptional humanitarian circumstances and that it would be unduly harsh to deport him.
It directed that he be granted a work visa, and a 12-month work visa was granted as a result, but that expired on June 29.
A further work visa under the partnership scheme was lodged by Lomu in May, but he was not issued an interim visa which meant he is now unlawfully here.
INZ manager Michael Carley said no one was entitled to an interim visa as a matter of right, and the decision lay at the absolute discretion by the Immigration Minister or Immigration Officer, who was not required to give any reasons for their decision.
Carley said Lomu's current partnership work visa application was being prioritised and further information had been requested to establish their relationship was genuine and stable.
A residence visa under the partnership scheme lodged by Lomu last September was also currently going through the verification process.
