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Parkes has had rail service from Sydney for over 100 years, although now days services on the lines in the central parts of NSW including Parkes are at best infrequent, with the majority of services having been replaced with coach connections from other lines.
Today only a single rail service stops at Parkes every week - the Sydney to Broken Hill Xplorer train service. Fares vary, but appear to start at around A$38 one way "saver" ticket (not available on all services), or $53 for a standard Economy fare. Daily services are available via train to Orange, Cootamundra or Lithgow with connecting coach services to Parkes, at similar prices.
The document here from 1996 states that the one-way far from Sydney to Dubbo in 1996 was $53, so likely Sydney to Parkes would have been around the same. Seasonal fares (eg, the "saver" fares mentioned above) did not exist at that time, so this would have been referring to the standard Economy fares.
The definitive answer to your question would have been found in this guide. I have no doubt I would have had a 1998 copy of that guide - just as I have no doubt it would have gone in the bin when the 1999 version came out!