I have about 9-10 days in Australia, I'm trying to squeeze in Sydney for about 4 days (2 sets of nights at the PH), and Cairns / Port Douglas for 4-5 days (probably PD because of the Sheraton) to get PADI certified, I'm trying to see if I can squeeze in Whitsundays as well.
Anybody did a quick stopover in the Whitsundays? I won't be able to do days of sailing there (with two toddlers) but would like to try and do a helicopter tour / scenic flight, especially the ones where you can hit the reef and stopover on Whitehaven beach for an hour or so. Problem is many of them seem to leave from a small regional airport that would be harder to get to.
I think 4 days in SYD over shabbas is fine (though MEL be more convenient, not sure if there is a shul within walking distance of PH SYD).
I don't think you'll need 5 days for PADI... especially if you take all the classroom stuff beforehand, which may be a good idea considering your time crunch.
If you can get your CNS time down to 3 days, with 4 days in SYD, I'd definitely trying to squeeze in HTI, especially considering there's a non-stop flight from CNS-HTI for 4500 Avios.
HTI is worth splurging $300 AUD ($215 USD) on a nice place for a couple nights. It's really like no other place I've been. So charming, what with the no cars allowed and some places giving you your own buggy to traverse the island.
We went fishing one day, went a full day sail to whitehaven beach, and then did a plane ride over the whistundays and heart reef. More than enough to keep you occupied for 3-4 nights.