While the Scottish Government pledged to improve post-diagnostic support in its national dementia strategy in 2010, the latest statistics showed just 46.7 per cent of patients were referred to specialist services.
Of this group, 83.9 per cent received the proposed year of aftercare.
Opposition politicians branded the figures "unacceptable" and "shameful".
An estimated 17,496 people were diagnosed with dementia in 2016-17, NHS data showed, with 8,178 of this group referred on for post-diagnostic support.
Almost three-quarters (72 per cent) of newly diagnosed patients in Shetland were referred for support but in NHS Orkney this only happened in 10.7 per cent of cases.
READ MORE: Brian Wilson: Scotland’s ‘free personal care’ is only free if it’s fundedNHS Grampian had the lowest referral rate of the mainland health boards, with fewer than a fifth (18.6 per cent) of people referred for post-diagnostic support.
Scottish Conservative mental health spokeswoman Annie Wells said: "The target set is that every person with dementia should receive these support packages for at least a year after being diagnosed.
"Yet despite this SNP pledge, fewer than half are being referred.
"And of those who are, not even all of them are getting the support they need.
"Every year the number of people diagnosed with this cruel condition increases, and that should be matched by improvements in care and performance.
"Yet on the SNP's watch, it continues to be completely unacceptable."
Labour health spokeswoman Monica Lennon branded the performance "utterly shameful".
She said: "Our health service has to adapt to the challenges of the future and more and more people are being diagnosed with dementia each year.
"The SNP has been in charge for more than a decade. They must urgently ensure people get the support they need."
READ MORE: Frank’s Law: Scotland’s dementia patients ‘dying’ awaiting careLiberal Democrat health spokesman Alex Cole-Hamilton said the target was being "missed by a huge margin" as he called on Health Secretary Jeane Freeman to "urgently establish why thousands of people appear to be missing out on the support they are entitled to".
The Lib Dem MSP said: "A coordinated package of support can make a big difference to individual patients and their family as they understand and adjust to a diagnosis.
"A plan helps people live well with dementia, ensuring they are accessing the right services and finding the best ways to manage their symptoms."