The Flying Doctors - Season 5 Episode 9 Bitter Harvest

Chemicals are being dumped illegally. A family returns home after a holiday to find crops are dead and stuff all over the place. The father has a row with the man that should've been looking after the property, it seems like he is drunk, they start fighting and the man gets shot and dies. The father goes off on his own on a horse for 2-3 days. The boy starts feeling sick and having trouble hearing. Chemicals are found to be in the water. The father needs to be found but no one knows where he has gone, he is eventually found.