The pilot of VH-OGN advised that whilst on descent approaching their cleared altitude of 5000 feet via a Hopla 1 STAR for runway 16, they received a TCAS proximity traffic warning with no altitude readout. The intruding traffic was sighted and was reported to be similar to either a Cessna 182/206, tracking from left to right and about one and one half nautical miles to their left. VH-OGN was levelled at 5400 feet and banked to the right. The other aircraft maintained its track and passed approximately half a nautical mile behind VH-OGN. The captain of VH-OGN said that while this was occurring, the TCAS changed to a "yellow traffic with aural warning". He estimated that the Cessna was between 4800 and 5000 feet. Although the Cessna was squawking code 2000, there was no altitude readout on air traffic control screens. Attempts to contact and identify the Cessna were not successful.