Mr McDonald said petrol thefts used to be common, but modern cars often had anti-theft technology to prevent fuel being siphoned.
He said thieves would probably target cars without locks on the petrol flap, or vehicles that were unlocked.
He believed the two thefts caught on video were linked.
A witness to the June 7 incident described the offender as male Caucasian in his early 20s, about 5'10" tall (175-180 cm), skinny build, shaved hair and wearing a black hoodie and black sports pants.
People in the Duncan St, Boydfield St, Moana St and Jones St areas are asked to be vigilant.