Hi there,
As mentioned, there are lots of decent bach options - we stayed at one in the older part of Tokerau Beach a few times (pre Covid) with an ideal flat section with plenty of room for boat parking.Tokerau is ideally placed for either the Doubtless side or the other side - but it's beach launching around there.
The Mangonui/Coopers or Taipa areas would be best if you need a ramp. Check that any place you rent at Coopers or Mangonui has suitable boat parking, as many of the baches have steep access as it's quite hilly.
You won't find many dirt cheap places, but there are quite a few reasonable ones. Obviously beach front or more luxurious, the more you pay.
The Whatuwhiwhi Top 10 is very highly regarded, and a guy called Real Kiwi Fishing/ fish whisperer on YouTube did a good video on the camp (presumably he got a bit of a discount for it, but it seemed genuine). He stays in a cabin. All set up for fishos with boat trailers.
One thing I'd check on with a bach is wash down facilities, if you are taking a boat. It's shaping up as a dry summer, and if they have a real drought you don't want to rent a place which has restrictions on hose use by autumn time.