Does anyone know any sites that actually offer this?

I can find various ones that offer cheapest flights between X and Y for a given month or year, but not a working multi city version.

With my kids mum passing away, I want to take them on a holiday to catch up with some family and family friends in Aussie. Don't have a huge amount of budget so price is a fairly major factor. Anyone know of a site where I can say I want to go from Wellington to either Brisbane or Melbourne, stay a week(ish), then to the other Aussie city for a week(ish) then home again. Time of the year and order of travel are irrelevant for my purposes.