I don't know anything about family law or courts but am another type of lawyer so know a little about how to make your case. Your reasons sound good but the way to make them more effective is to group into categories and then back up with examples e.g.
- Disruption
(including Disruption to their routine and primary residence; The knock on effect to their education; They have never been away from me for longer than two weeks; DD1 has previously been upset on the phone to me when away from me in the UK. )
Their lives have already been disrupted by the divorce and change in life, while you are doing your best to provide them with consistency you are concerned that such a major change may do more harm than good. Then suggest a compromise e.g. wait until they're older, or if ex really wants this he should pay for your flight and accommodation as well so that you can be on hand and perhaps they can stay with you (is that possible with work situation?). Given he's not had too much contact with him you may be able to make an argument for how this is the best way to enable access.
2.Certainty
(Including Will ex take the time off work to spend quality time with them or will they be looked after by a third party/ his partner / his parents whilst in Aus.; Where/when is he going to Aus? What is the house like in Aus ... where will they sleep? (Point out importance of children knowing what's happening, give examples of his unreliability and constant changing of arrangements and passing them off onto relatives not seen frequently.)
3 Fairness and importance of home life
I would like to spend time with the children during the summer holidays. They may also want to see their friends go on camps, play schemes etc.
What you are trying to emphasise is that you are giving a solution in the children's best interests and can be relied upon to do so.
As said I don't have experience of family law but if you present an argument in a clear way with enough good examples hopefully the right result will come through.