VIC Child Support Increase When Ex is Pregnant?

Also, how do we prove that she is still claiming single parent while living with the new partner? You don't. It has nothing to do with you... It doesn't affect child support. Pick your battles.

His child support will go up while she isn't earning and worse. When new child arrives that will cause him to pay more. No fun so far right...

Care % is problematic, but you can win this one... You can't win the other two questions.... Keep a diary, make sure communication about extra time is via text so there is proof, more so than verbal communication. Do that for a few months. Then ask her if she would agree to change the care based on the reality. Offer to mediate to get a new agreement that reflects the actual care... Then provide the diary and text message to CSA and ask them to re-assess based on that information. You should win that one

Re the child support going up, my reading is that the baby won’t be his. If that’s correct, his child support won’t go up on that basis.

As to the care percentage, there are brackets and care percentage does not equal cost percentage. Check out section 2.4.5 here: 2.4.5 Care, Cost & Child Support Percentages | Child Support Guide

There’s a table a short way down.

Yes, I think it will and I think once the child is born she can claim she is responsible for 2 dependent kids and madness I know, but that will result in his child support also going up...

You only pay child support for children that are yours (rather obvious, but bear with me). When the ex ceases work, their income will drop - which will affect the calculation. They will have a multi-case allowance applied to their taxable income - which will also affect things, but given they're not working it should have minimal to no effect. It's possibly likely, for example, that their government benefits will place them close to the self-support allowance - putting their adjusted taxable income for child support purposes at or close to zero.

It's then a factor of what bracket their combined income puts them into - a drop in the ex's income may have the effect of dropping a bracket, for instance. Then the determined amount of child support payable per annum is multiplied by the cost percentage (derived from the care percentage).

So, his child support payments will go up from the ex ceasing work - but the ex's new child will likely have negligible to no effect on its own.

I would focus on modifying the child support to reflect the fact that your partner has 50% instead of 27%, as this is exactly what payments are calculated on. If your partner has the evidence to show he has the child at the 50% bracket (texts, emails, well kept diary) he can apply to modify and reduce child support, although he may want to consider a discussion with the ex before filing without notice as the agency will contact her, and if there is any issue of conflict or the ex not agreeing to the modification this may cause tension.

If the ex is likely to be fine with the change and he is wanting to have payments reduced maybe he could talk to her about it beforehand. If it's likely she will deny he is actually at 50%, you could just go ahead and apply for the modification but expect some fallout from her and expect to have evidence or it won't happen.

The other two issues are out of your hands. Possibly the increase due to the ex not working might be slight, but you can't do anything about her allowances and centrelink arrangements.