How do I oil it. I bought it new and it seems to stitch losely.

If you haven't removed the top to clean and oil the cams then likely they are stuck, hence zigzag.

Hi,
I have several of these machines and they all have the problem you described. It would seem that the lubricant used by Viking has dried out with age and turned to something like varnish. If you can remove the back plastic cover without breaking it, then you can gently heat up the internal mechanism (hair dryer?) and oil, exercise the zig-zag linkage.
Be careful of the plastic, it is old and brittle. Sometime just turning a screw loose will break it, making reassembly difficult.
These are great old machines but sometimes you have to be satisfied with only a good straight stitch.
Bob

Difficult to diagnose. Try a brand new needle that is compatible with the thread and fabric.
All About Needles

If the machine is mechanical, it may need cleaning and oiling if the decorative stitch mechanism is sticking.

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Is it a new 830 computerized model or the older 830 Record mechanical?

If it is the 830 Record, it needs oiling--1-2 drops of good quality sewing machine oil (specifically concentrate on the stitch selector lever and oil the metal parts that move against other metal parts) Do NOT oil the cams, plastic gears, any rubber parts, ie belts, bobbin winder wheel.
Bernina 530 through 830 Record Oiling Diagram

Your machine is an older mechanical. It needs cleaning and oiling to keep it going. It is probably seizing because the old oil has dried. Continued use could damage the motor.

Wheel on right side that controls the needle up and down is very tight...

as for a manual try these sites
sewingpartsonline.com
sew4less.com
sewusa.com
ebay
is the needle the correct one for the fabric, if your overalls are denim
the you need to use a denim needle, flat side goes to the back of the
machine.
as for a sewing machine shop, in your browser type in sewing machine
shop in NZ & see what comes up.

Keep checking all internal moving parts and keep them well oiled. I believe the machine has just sat for some time and needs to be run in to loosen it up again.

I needed to have it fixed as soon as poss so I had to take it to a machine repair man. He saw straightaway that it had a broken gear which was something I could never have sorted myself. Thanks for your reply.

Be sure you are in the correct stitch pattern...if you are on a buttonhole stitch or a stretch stitch it normal for the machine to sew backwards. Be sure your length is to the right of 0 and not in a strtch stitch model or buttonhole mode.

Bonnie