Shared parental responsibility is assumed and probably stipulated in your orders. So any major decision requires both parents to agree. Relocating interstate fits the description of major decisions.... Now I'm guessing dad won't agree? Then your only option is court...
Chance of succeeding: no idea...
But I'll give you my scenario to ponder... 5 nights a fortnight with me and half holidays done via consent... The orders were less than 2 yrs old when the ex decided she wanted to move 6 hours away to be with her new man.....
I refused to agree for the kids to go... She sought legal advice and I got some pretty intimidating letters from her new solicitor basically telling me I'm wasting my money trying to stop her. I wrote back to the solicitors with similar gusto to let them know I would not agree and I would apply to court in the event that she took the kids...
So I found it intriguing that in the first instance I was informed that her leaving was a foregone conclusion and the consent orders / shared parental responsibility mean nothing...And with hindsight it really did knock my confidence in the legal profession, given her solicitor was basically advising me to give up on seeing my kids as frequently as I had for the past 2 yrs. Fact is her solicitor was trying to intimidate me into giving up. In the end the ex decided to leave but the kids stayed with me.... Happy ending as far as I'm concerned...
Look no -one on this site can know your situation better than yourself... If I was my ex's mate and I was advising.... My opinion would go like this: What are the odds that he will apply to court to have you or the kids returned? And what is the chance of you actually winning? And are those odds worth betting on? because it will be an awful waste of money to move, then have a legal bill and then have to choose to either move back or send the kids back to dad?
So in my case the ex would have known in no-uncertain terms that I would have applied to court and that the odds were against her actually winning.... If I was to hazard a guess, I reckon you're in a similar situation.... And in dad's defence.... He has (possibly) had a bit of a struggle to get an agreement out of you for the 4 nights a fortnight....
So if you look at my scenario - it was a hard slog getting any time with the kids... I had to basically sell myself out on the assets - I bought the opportunity to see the kids by making a generous asset division as part of a proposal that gave me 5 nights a fortnight... Prior to that me seeing the kids was dependent of her moods and frankly it really really sucked to be in a scenario where I only got to see the kids IF it suited her and that I had no capacity to argue because any argument would see her refusing me access to the kids for a few weeks....
So having finally sorted fair consent orders that clearly defined my time with the kids, the thought that the ex could up and move and those consent orders be all for nothing? no way is that fair and frankly, I reckon your ex might have good reason to think the same...
Why? Well in your first post you mentioned that your reasons are economic and family... As allforher pointed out the children are part of the father's family too and you will be removing his family from him.... Your extended family don't have priority over the relationship of a parent with their kid....
Economic... Again, this is very specific.... So if you are a marine biologist, obviously jobs are near the water... If you currently live in Broken Hill there isn't much need for marine biologists... So that isn't a great sales pitch either (but again, without knowing your line of work, i can only assume...)
Final thought - risky warning warning - Call his bluff... So let me give you my partner's scenario.... She wanted to move, they had one kid together... She took it to court and applied for relocation... She said she was happy to leave the child with dad if that was what the court ordered but she was leaving....
Magistrate asked dad how he would respond and he said he was not prepared to be the primary carer of the child. So she got to leave with the child... But that is a high risk throw of the dice... Not the sort of thing I'd do... But her ex is not the most paternal... It only took a few years before he stopped seeing his daughter as per the orders....
So I think my post has given you some food for thought - but you're not gonna get a definitive answer here...best you can do is apply to court, or leave the kids with dad, or accept the situation is what it is... that is the long and the short of it... true?