Get a price on the net ... then ask your travel agent for a quote on the same cruise.
I found the net is better with more information and I always ask for the actual cabin I would like from the deck plans.
Which deck .. I look for the deck with balcony with cabins above and below me ... not a deck with bars etc above and do not get below the walking track.
Being in Australia I also have to get airfares and they can now also be cheaper on the net ... guess it's the world we now live in one has to check around for the best price.
The only one I know that will not let us book overseas is Princess.
Must admit I would give nz.cruise.co a go .. I just asked them for a quote on something.
The only trouble witha cruise .. when you get the bug you will never stop .. they are great value and show you the world.
If it's your first I would recommend a TA who specialises in cruises. Plenty on the net & usually a freephone number to talk through your particular requirements. i.e kids/no kids/formal/informal etc. Most High St TA's will tell you they have a cruise specialist but buyer beware, how many have actually been on a cruise??? A freephone number at the top right of this page may be a good start point.