Child support wasn't a common thing in the 70's in Australia, most women didn't seek it as it was a lengthy, expensive and often embarrassing court process. Remarrying was a more common way to be financially supported. The child support scheme was introduced in 1988 in Australia.100% she knew too much.
Just speculating, but a likely scenario is IK was asking for child support or for him to return whatever he stole from her, or she would report him. He would rather kill than pay up.
Another scenario is police started asking questions and he knew she would be honest, so she had to go.
History of the Child Support Scheme | Department of Social Services, Australian Government
Improving the lifetime wellbeing of people and families in Australia.
www.dss.gov.au