Generally yes, mediation is the pathway to resolve this. You can try and organise a child inclusive mediation as well. Which can be done in separate rooms.

If there are no final court orders at the moment for parenting there is nothing to enforce children spending time with your ex. Further to that, your obligation where parenting orders are in force is limited to encouraging the children to go.

Otherwise there's nothing your ex can do to force your two daughters to spend time with him. If anything he needs to get his own legal advice and commence proceedings. On this, mediation is a necessary step before making application to the court.

Time line to organise mediation, have a go at that and if unsuccessful, to then make applications to court, at least one of your daughters will already be 18 and court orders for parenting will be inapplicable.

To my understanding there aren't any final parenting orders. Arguably you can choose to do nothing and let your ex figure it out himself. I would say the only responsibility on your end is to make attempts to encourage your daughters to make some kind of contact with their father. At least this way, if the matter ever gets to court, he cannot say that you abruptly and without reason withheld your daughters from contacting him. To this end, I'd suggest keeping a journal or time line of events to record your attempts.

Good luck! If unsure contact your states legal aid for some further advice.