Sounds more like cable steering can you put a pic up of where the steering connects to the outboard

Air maybe a problem, bleed the system.
Check the the motor can move easily when detached from pivot - ram arm of the hydraulic unit.

There are also different vicosities of hydraulic fluid. This may also need to be checked or replaced.

I assume you have no leaks in the system.
Also is the unit rated for the weight of the motor.

Leaks and/or problem with one of the non-return valves in the helm pump?

Can combine most of the above.

Any leaks?.. or where there any leaks?..and now little fluid left.?

If old change the fluid.. hydraulic fluid over time absorbs water.. then forms acid, the acid corrodes the bores... rather than wear out, its lack of care and maintenance... this also tends to corrode valves and damage pumps. Stainless certainly prevents much of this

The system  may need bleeding/ purging, espec if change fluid or fluid is low...

Being old, you may also have an inner hose liner break down, and be stuck in a valve.

Does anyone know a good honest and cheap boat repair shop near Mt Albert, Auckland that could give the steering a refurb?

 Bottom line, espec when comes to hydraulics there is nothing that is cheap...except for most seals etc... but even then dont go real cheap, there is a big difference between elcheapo seals O rings etc and quality.
hydraulics are not rocket science... often a couple specialist tools, and a manual to figure out a few bits .....what is specialist thu is crimped fittings.. they are not cheap.

 1st off, change fluid, bleed/ purge.. check seal leaks etc.and then take from there..

I had a similar issue with a 5.7m Ramco. Helm would turn one way easily but just didn't want to turn the other way. It turned out the cable sheath had a split in it and the inner cable would bulge out the side instead of turning the motor. A replacement cable was cheap (about $140) and easily fitted.

Hi, I used to do cabling manufacture for a living, probably broken inner cable spiral. Will work pulling but not work pushing. I have spent countless hours stripping , greasing and cleaning steering cables to have them only last a couple of months afterwards then fail, very dirty job with no reasonable outcome. They are cheap now , compared to what they used to cost ($500.00) around $150 for new. You wont look back, just don't install with any sharp bends and not on the floor.

Just look for the numbers printed on the cable shealth, some are printed on and others are heated in. Normally one clamp to undo at helm, wind old one out, wind new one in and clamp, just grease the helm with some water proof grease. Don't pull new cable apart to grease. The number will denote cable type and length.

Have a look at this video - it might be useful.

Seems to be quite a good channel with lots of tips and tricks.

Why muck around with a crappy set up, chuck it out and put in a new cable, cheap insurance.

I have hand this problem before .You either have the old Morse or a Teleflex cable. You can strip the the stainless steel sliding tubes ,remove the cable from the boat & off the motor its easier.Pull the inner stainless tube out as the inner smaller tube slides in the lager tube .Clamp the inner tube in a vice & and gently cut on the crimp with a hacksaw cut a little at a time and turn the tube as you cut , careful there is a 6mm rod inside the inner tube just cut the crimp only go to a depth of +/- 1.5 mm around the tube . Once you've through the crimp push the inner tube back into the outer tube this will then expose the 6mm rod at the end of the rod there is a lock nut , undo the nut and turn the Lug off the 6mm rod its threaded . Then from the other end pull the cable out .clean the cable casing and the inside of the outer stainless steel tube . once you've done this put every thing back together like it was then Tig / fuse weld were you cut on the crimp back together . You have a choice spend 150 dlrs for a new one or 10dlrs for cleaning material and your cable is as good as new . Another thing to look out for is the slightest bend you have on the inner tube will cause the problem too . While you have it apart place a straight edge on the tube and check it.