We have a weed spreading rapidly it has been identified as Richadia have used Metsolfuren which only kills it temporarily but also kills the grass ....we are looking at a 5 acre patch

Weed Control Services Canberra, Having lawn weeds is a normal and expected part of having grass. Once you have lawn weeds they can spread quickly so the sooner you get on to treating them the better!
Most weeds can be easily controlled by applying a selective herbicide which will kill the weed but not harm the grass. Depending upon the degree of infestation repeat applications may be required. Weeds such as Bindi are best treated before the plant sets seed and can be dug up and removed by hand.
Lawns that are dry, compacted and in a poor condition will be more susceptible to weed attacks. If you have any areas of lawn where there is damage or there is dead or dying grass, replace the turf to stop weeds from taking over the patch of ground.
The easiest way to prevent weeds from taking over your lawn is to keep your grass in a good condition. This involves ensuring your lawn is getting the required amount of water through natural rainfall and supplemented with watering in times of drought. Regular fertilizing with slow release fertilizer at least once a year will assist in keeping the lawn healthy. Regular mowing will also help to remove weed seed heads before they can mature and seed your lawn.

One quart of vinegar -- with 20 percent acetic acid -- mixed with 4 ounces of lemon juice will kill the above-ground portions of the weeds and grass. The foliage, flowers, buds and stems will wither, dry and die, but the weed's roots are left intact. For a tougher weed and grass kills, combine 1 tablespoon of 20 percent vinegar, 1 tablespoon of gin, 1 quart of water and 1 teaspoon of dish soap. This mixture kills the above-ground and below-ground portions -- including roots -- of the weeds.

Any round up mix that will kill the weeds will kill the grass. the only way I can think of is to tie a sponge to a stick dip in round up and apply to each weed, you will not have to cover the whole weed, just touch a few leaves.

I would not use Preen in your pond. Preen is a pre-emergent and needs to tie up in the top 2" of your soil. It does not kill existing grasses and weeds, only the germinating seed as the meristem evolves. Use a product specifically designed for aquatic use.

last bottle of this product I had , it had a lift off label that included all the mixing instructions ,

I will say if you have long fence lines or large areas of grass/weeds to kill , id use something a bit more economical as this product /it works well but it is a bit on the expensive side for large areas , ive found that just a block of softener salt soaking in a five gallon pail of water for a few days , works very well for large areas of land sprayed on to clear it of weeds and grass (although it's not rain proof)

for roundup weed & grass killer concentrate plus you would use 3 oz per gallon or 29.57 cm^3 per litre

Hi , I would be very carefull around good plants. Roundup will kill almost any plant, including aquatic plants, so you want to be sure to avoid spray drift onto other plants or into water.

Very important to buffer the pH of the water at pH 5. Then add Roundup /Glyphosate 360 at a 2% for grass weeds and 3% for brush weeds. Ensure that the grass is dry when treating and no rain is expected within 6 hours. Please take care with roundup as it is poison .

round up i think has a two week withholding period, so wait two weeks & turn the soil over & re seed is the only way I can see