we had 14 nights to cover both islands over easter this past year...

can't comment exactly on your desired sights (we didn't do coromandel or abel tasman)

our itinerary was as follows (and we found to to be about right in terms of travel, despite a couple long travel days). NZ is so far away, we wanted to make sure we saw as much as we could, in case we don't make it back (having been there now, i'd say there is little risk of that)...

- auckland (2 nights), 1 day spent on waiheke (very cool)

- drove to tongariro to do the crossing (2 nights)

- drove to the coast for wine tasting in hawke's bay (2 nights)

- flew from Napier to Queenstown and drove directly to Milford Sound (1 night there, kayaking the next day)

- spend 2 more nights in fiordland tramping during the day

- drove to otago (arrowtown) and spent 2 nights wine touring

- spent 2 nights in wanaka, wine touring & tramping the rob roy glacier

- flew back to auckland (1 night at the airport before our flight the next day)

given a choice, had we had a few more days, we probably would have added one more area ie: marlborough / the west coast glaciers / abel tasman.

with 17 days, you might have enough time to do something like this... (i don't think it seems like too much travel)

- auckland (i assume you're flying in here) (2/3 nights)
- coromandel (2/3 nights)
- tongariro crossing (2 nights)
- fly from taupo to nelson, abel tasman (3 nights)
- fly from nelson to queenstown / fiordland (5 nights)

then you've got a couple extra nights you can allocate where you want depending on travel schedules.