I agree that they must have known each other in Belgium in 1968. They could have lived together.
IK tells WW that she is emigrating to Australia and has her paperwork approved.
WW decides that he will follow her to Australia because if he is married to her, he can get AU residency too.
He joins IK in Sydney with a faked marriage certificate from Belgium.
Exactly nine months before their child was born in mid January is mid May 1969 a week before they arrived.
IMO IK could have been pregnant either just before she arrived or just after even in June depending whether the baby was delivered on or before her due date.
What I can't agree with mainly is the fact they they both leave on the same date and the same plane in Feb, 1970. Is that just a coincidence? If you don''t want anything to do with someone again, you wouldn't travel on the same plane to the same place as them, would you? I wouldn't.