If the OP was in Chch, I've got a portable bench saw with narrow kerf ripping blade they could borrow - or bring the timber over and I'd cut it in a few minutes.

If using a skilsaw, then presumably the timber is going to be screwed to joists, so pre-drill and rebate at least some of the holes before ripping it down, use those holes with decking screws to attach to some offcuts of timber which are firmly attached to good solid sawhorses (so there's clearance under the decking timber without demolishing the saw horse - and you're not trying to cut over the end of the sawhorse -but closer to the centre so it's more steady). Some hardwood decking timber is "watershed cut" (not sure if that's the right term) so it's got a convex top surface which may cause some grief trying to hold a skilsaw flat and steady.

Ge0rge: If you want to make decking timber thinner, a table saw would probably be best. Expensive to buy for a one-off (unless you like collecting tools!) However your local menz shed will more than likely have one that you might have to drop some biscuits off to make use of.

It can be done with a circular saw. Correct setup and clamping will let you finish with all your digits still intact, but a table saw is designed for this sort of thing.

Edit - beaten to the punch by @MDF

Clamp it to your sawhorses and use the fence attachment on a circular saw and you should be able to do an OK job.

Bench saw, take it slowly, watch the fingers and push the last foot or so through with a stick.

Thanks all for the suggestions. Cannot get wife approval for a bench saw for a one off job so will try my luck with a circular saw. Not sure why i thought i could do this with a jig saw  :p

kryptonjohn:

Clamp it to your sawhorses and use the fence attachment on a circular saw and you should be able to do an OK job.

Use a wood thicknesser, No idea why you'd want to rip it with a skilly.
Devonport have 2 in their local woodwork shed.

If you're game - and CAREFUL - , you can build a basic jig to turn your skill saw into a table saw - see for example

If you want to have a crack with a skill saw and your boards are longish (>1.5 - 2m) and you don't have a dozen saw horses, I'd be inclined to screw it to your decking joists along the length of the keep side. This will keep it really stable and give it plenty of support. Obviously adjust the skilly's cut depth so you're not sawing deep into your joists (or use spacers). Actually screwing the decking board down helps since they're usually not wide enough to clamp and have a fence on top. 

Again, if it's longish, I would try and set up fences along both sides of the board the width of your skill saw. If you only fence along one side, the blade can walk away from the fence. Even if it walks across the discard side and doesn't spoil your keep side, you still have the devil's own job getting it back again without it deflecting the next time you try and cut. 

If you're shopping and the budget will stretch, have a look at the Toolshed's rail saw (you can get it for $299 when they're having a sale). While you also need to add rails to the purchase price, the rail means it can't walk either way and you don't have to mess around trying to find a straight fence.

Circular saw with a fence should be fine for a one-off. Just take it slow and sometimes handy to have someone just hold the timber for rather than do it all yourself.

And if you wanted to be a guinea pig for the rest of us, I think you can get a Ryobi skill saw track. Unfortunately Ryobi doesn't supply this locally and won't confirm or deny anything when asked.

But:

This track set: https://www.amazon.co.uk/Ryobi-RAK03SR-Rail-RWS1250-G-RWS1400-K/dp/B00CS2MUJC is listed as compatible with the RWS1600-K. Google image searching that shows it looks pretty similar to the NZ spec RCS1600-G that is available here, including the slot (though no lazer).

Tagging @richms who is a fellow traveller on the Ryobi road to bankruptcy and may know something more.

However your local menz shed ...

t0ny:

 

Not sure why i thought i could do this with a jig saw  :p

 

A grunty jigsaw would rip lighter pine boards - but it wouldn't be fast or fun.

The trick would be securing the piece while leaving room underneath for the jigsaw blade and also being able to run the jigsaw along the top without hitting clamps or whatever.

BTW you can sometimes hire saw benches.

t0ny:

Not sure why i thought i could do this with a jig saw  :p