Active job

Plumb washing machine and sink to outside trap

After realising the sink fills up with water when the washing machine drains (might have been doing this a while) we had some drainage people out who couldn't really find the issue. It drains to a 40mm copper pipe under the sink and at best guess drains to the copper pipe outside in the bathroom trap, it’s the only one I cannot account for water coming out of, not a single drop comes out, although shoving a hose pipe up about 3 meters seems to bring out a bit of water that smells like washing machine water. The sink drains very slow so it might be making it out somewhere else. The drainage people have said it might be corrosion but not sure, they can stick a camera up but that’s a lot of money to just find out we need to replace a pipe or similar. Nothing else has an issue, no signs of issues of back flow to any sink, shower and so on. I’ve put a full packet of Draino down the 40mm pipe but it didn’t do anything. I guess the best option (unless someone knows what it is) is to plumb the sink and washing machine along the back wall and outside and cap the 40mm pipe off. Looking for a quote for this job and we are located in Tarnica Road, Northpark, Botany.