Around chimneys and other roof penetrations lead flashings are traditionally used to create a watertight seal, but if poorly maintained lead flashings can cause major problems for tank water systems. There are now some safer alternatives.
https://wakaflex.com.au/wakaflex-waterharvesting/

Wakaflex is Australia’s leading non-lead roof flashing tape suitable for nearly all roof profiles and types including tiles, metal and polycarbonate. Wakaflex is a safe, non-toxic roof sealant and is quick and easy to install with no specialist tools required.
https://wakaflex.com.au/product-features/

Established in 2004 (under the Combined Group of Companies) we are a leading national supplier of innovative building and roofing products throughout Australia. Evobuilds lead-free flashing range is the most comprehensive available in the Australian market and directly reflects the relevance of this growing category.
https://wakaflex.com.au/about-us/

Wakaflex is Australia’s leading lead free roofing flashing tape. Choose the safe, durable and easy to install roof flashing on a roll for tool-free installation.
https://wakaflex.com.au/