Waterco sand filter has a sow discharge of water at the waste when the filter is runnin normally

Leave 10"-12" empty space above sand (freeboard).

Www.waterco.com

That increase is due to the filter loading up with debris. Is the pool unusually dirty? If there is a lot of dirt and silt in the pool use the "waste" setting on your backwash valve while vacuuming.
This allows the water to bypass the filter and go directly out the waste line. Keep an eye on your water level because you will be removing water from the pool and may have to add more.

Reference your pressure gauge as the 12:00 position. Going clockwise-
12:00 is Filter, normal running position
2:00 is Waste, water goes from pool and exits the waste line, bypassing filter.
4:00 is Closed, used only when servicing equipment
6:00 is Backwash, reverses water through the filter and goes out the waste line.
8:00 is Recirculate, water bypasses filter and returns to pool.
10:00 is Rinse, water goes through filter and exits to waste line, used briefly after backwashing to clear sediment from filter.
Note that the "pointer," not the handle that you grab, indicates which setting you are on.

When the pump is running is water discharging from the waste pipe outlet?
If so then the selector valve that switches the pump outlet to waste, backwash, filtering etc. is faulty.

When the filter get clogged the pressure in the filter increases and there is less water flow returning to the pool.
If your filter is a sand filter, when you turn the valve to backwash and turn on the pump it reverses the water flow in the sand filter to flush the dirt out of the filter to waste. Look at the window openning in the waste pipe and keep the backwashing goinging until you see clear water.
Then turn off the pump and move the control valve to Rinse, turn on the pump for a few minutes, this returns the water flow back to normal but the water goes to waste to make sure the sand filter is clean.
Turn off the pump and move the control valve to filter and turn on the pump. This mode is the normal operating mode and the pressure gauge in the sand filter will show a drop in pressure.
Some pressure gauge shows a red section to indicate when backwashing should be carried out.

When backwashing with this valve, it is supposed to come out the waste line. Also when in "rinse" and "waste" it will also come out the waste line. It is acting just how it is supposed to act.

Replacement of the sand is easy but there are a couple of things you want to do first.
Make sure you have a good shop-vac, get gloves, and purchase the proper sand.
This sand filter calls for #20 silica filter sand. (this will be available online, at your pool store, or at any water treatment facility).
Here is the link for this filters manual, and for the brochure (which has good pictures)
http://www.waterco.com.au/CMS/uploads/brochures/ZZB0512_Thermoplastic.pdf
http://www.waterco.com.au/CMS/uploads/Manuals/Sand_manual_829006.pdf

The manual has step by step instructions on filling and starting the filter.
The only part it doesnt tell you about is how to empty the old filter media.

To start this:
1. turn off circulation pump
2. Bleed any pressure or vacuum from the system using the bleeder valve, or just rotate the multi-port valve.
3. Open the drain valve at the bottom of the filter if there is one (some dont have one)
4. remove the collar between the multiport valve, and the tank. (this should be 2 phillips head screws)
5.Remove the multi-port valve.
you now have access to the sand.
6. Remove sand by whatever means (either a scoop or a good shop-vac with a thick hose)
be sure to be carefull once you get 2/3 of the way down the inside of the tank. There are LATERALS down there that are most likely brittle from chlorine.

Once sand is removed check and clean the laterals at the bottom of the tank.
Clean and flush the filter body and follow the directions in the manual to fill and start-up the filter.

If this helped, please vote positively for me.